RV Toilet: Why Nobody Publishes the One Measurement You Need

Replacing an RV toilet looks like the simplest job in the trailer. Two bolts, one water line, one seal. The part that catches people out is not the work; it is finding out whether the new one will sit on the flange the old one came off.

This guide covers what actually determines fit, what manufacturers publish and what they quietly do not, how plastic and ceramic bowls differ in practice, and five toilets worth considering once you know which one your floor will take.

Quick answer. Almost every current RV toilet mounts on two bolts through a three-inch floor flange, and on that basis most of them are interchangeable. Neither Thetford nor Dometic publishes the bolt spacing, so nobody can promise you a swap in advance. Measure the flange before you buy, and treat every claim of universal fit as a claim rather than a specification.

What decides whether an RV toilet fits?

Four things, and only three of them are published.

The floor flange. Dometic specifies a flange with a three-inch inside diameter, between 0.25 and 0.44 inches thick, with the toilet held by two T-bolts that run perpendicular to the back wall. Torque is 30 to 40 inch pounds, which is hand tight and then a little, not as tight as it will go. Thetford’s installation manual asks for the flange to sit 7/16 of an inch above the finished floor and supplies spacers to get it there.

The rough-in. This is the distance from the center of the flange to the back wall, and it decides whether the tank lid clears. Dometic publishes 10 inches for the 310 and 8.5 to 8.75 inches for the 300. Thetford publishes no rough-in figure for any model.

The water supply. Thetford asks for a 3/8 inch inside diameter line delivering at least 2.5 gallons per minute, connected with 1/2 inch NPT fittings. Dometic asks for 3 to 5 gallons per minute, with 2 as the minimum, at a static pressure between 30 and 100 PSI. A trailer that already runs a toilet meets this. A trailer being converted may not.

The bolt spacing, which nobody publishes. Both brands use two bolts. Neither states the distance between them anywhere in a manual, a spec sheet or a product page. Dometic’s closest published figure is on the 400 series, which is advertised as fitting rough-ins of 7.625 inches and up. That is the flange to wall measurement, not the bolt centers, and it is the one number on either website that looks like the answer and is not.

So the advice repeated on every forum, that RV toilets are all interchangeable, has nothing behind it from either manufacturer. It is usually true. It is not documented, and if it fails you find out with the old toilet already off the floor.

What to do about it. Before ordering, lift the two bolt caps at the base of the existing toilet and measure center to center between the bolts, then measure from the center of the flange to the back wall. Two numbers, two minutes, and they turn the whole question into arithmetic.

Plastic or ceramic bowl?

The bowl material is the choice that changes how the toilet feels to use, and it is genuinely a trade rather than a good and bad option.

Polypropylene. Thetford’s Aqua-Magic VI weighs 10.75 pounds and the Residence 12.3 pounds. Dometic’s 300 is the plastic model in its gravity range. Light matters more in a trailer than it sounds, because the toilet sits on a floor that flexes, and because you are lifting it into place over a flange while lying in a doorway.

Ceramic. Dometic’s 310 and 320 use what it calls vitreous ceramic. Thetford’s Style II and Style Plus split the difference with a ceramic bowl on a polypropylene base. Ceramic cleans more like the toilet at home and holds less staining over years, which is the honest reason most people who have lived with both prefer it.

Neither manufacturer publishes a weight for a ceramic model, so the weight penalty cannot be quoted. Dometic publishes no weight for any of its toilets at all.

How much water does an RV toilet use?

Thetford publishes one quart per flush, and sets it against the 1.6 gallons a household toilet uses. That figure is the same for the Aqua-Magic VI, the Residence, the Style II and the Style Plus. There is no published difference between them.

Dometic publishes no per-flush figure for any model. Its manuals give supply flow rate and nothing else. Retail listings quote pints and quarts for Dometic toilets, but those numbers do not appear in Dometic’s own documents, so this article does not repeat them.

Which means the water-saving argument between one RV toilet and another is mostly invented. The real variable is the person holding the pedal. A pedal held down for four seconds uses roughly four times what a pedal held for one uses, whichever brand is on the bowl.

Can you put a regular toilet in an RV?

Neither Thetford nor Dometic publishes a position on this. No manual addresses it, no FAQ answers it, and the silence is worth reading as caution rather than permission.

What can be said from what they do publish is that the two devices solve different problems. A household toilet keeps standing water in a bowl and a tank, which is weight that moves and water that sloshes down the road, and it flushes with 1.6 gallons into a tank that holds perhaps forty. An RV toilet holds a small amount of water above a sealed ball valve and empties straight down. Dometic’s own marketing calls its toilets residential size rather than suggesting a residential fixture.

If what you actually want is the feel of the toilet at home, that is what the residential height and residential seat models are for, and they are the same price as the standard ones.

What about a composting toilet?

A composting toilet is not a better version of the same thing. It removes the black tank from the equation, and in doing so it takes on a different set of chores.

Nature’s Head publishes 60 to 80 uses before the solids bin needs emptying, which it describes as about a month for two people, and its FAQ gives the more useful test: when the handle gets hard to turn, it is full. Liquids go to a separate 2.2-gallon bottle, which is the part nobody mentions, because that bottle fills in a couple of days and someone has to carry it. The unit weighs 23 pounds empty, needs no water connection at all, and runs a fan on 12 volts drawing 0.07 amps, which the manufacturer says should run 24 hours a day.

The warranty is five years, which is more than double what either RV toilet brand offers. The purchase price is roughly five times a good gravity toilet.

One caution on the published figures. Nature’s Head states five feet of 1.5-inch vent hose in its specifications and a four-foot hose in its FAQ, where it also recommends 1.25-inch PVC for a permanent installation. The manufacturer disagrees with itself, so plan the vent run after the box is open rather than before.

Five RV toilets worth considering

Every specification below comes from the manufacturer’s own manual or spec sheet. Where a figure is missing from this list, it is missing because the manufacturer does not publish it, not because it was left out.

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What did not make the list, and why

Two products sell well and were left out on purpose.

The Dometic 420 Platinum. It is sold widely under that name and Dometic has no product page for it. A 420 appears only inside the title block of the 400 series operating manual, with no dimensions and no specification of its own. Platinum appears to be a retail name rather than Dometic’s. A product this article cannot check against the manufacturer is a product it will not recommend.

The low cost porcelain imports. They carry more reviews than some of the picks above and they may well be fine. There is no manufacturer documentation to check any claim against, which means every figure in the listing would have to be taken on trust. On a page whose whole value is that the numbers came from a manual, that is not a trade worth making.

What fails on an RV toilet, and when?

Almost always the seal, and almost always in the same way. The bowl stops holding water between flushes, and within a day or two the bathroom smells of the black tank because the ball valve is no longer sealing it off.

Thetford recommends replacing the lip seal every four to six years. Dometic publishes no interval at all and describes the cause instead: mineral deposits from hard water build up on the flush ball or the seal and hold it open a fraction. Both are treating the same failure, one with a calendar and one with a symptom.

A seal kit costs a fraction of a toilet. If the bowl will not hold water, replace the seal before concluding you need the whole fixture.

What toilet paper can you use?

Dometic states it in every gravity flush manual: flush only water, bodily wastes, and rapid-dissolving toilet tissue, and it names paper towels specifically as something to keep out.

Thetford publishes no general guidance at all. Its owner’s manual has no toilet paper section. What it has is a product line of its own fast-dissolving tissue, described as breaking down quickly to reduce clogs. That is marketing copy for a product, not an instruction about what else may be used, and it should not be read as one.

So one manufacturer answers the question, and the other sells you something instead. The instruction that exists is Dometic’s, and it is sensible to follow whichever brand is on the floor.

Where the work stops

Swapping a gravity toilet is two bolts, a water line, and a seal, and it is inside what a careful owner can do. Three things are not.

A flange that moves. If the flange rocks, or the floor around it is soft, the problem is the floor and not the toilet. Bolting a new toilet to a rotten subfloor hides it for a season.

Anything below the flange. The pipe from the flange into the black tank is not a seal to be tightened from above, and a leak there is a tank job.

Macerating and vacuum systems. Those carry pumps, wiring and their own control logic. Nothing in this article applies to them.

Quick Answers

Where to go next

The toilet is one end of a system, and the rest of it has its own guides here.

• RV sewer hoses, which is what the black tank empties through, and where twenty feet in two sections beats one long hose
• RV water pressure regulators, since both manufacturers publish a maximum static pressure the toilet valve is built for
• Winterizing an RV, because the toilet valve and the seal are among the first things a freeze finds
• RV water filters, given that mineral deposits on the flush ball are what Dometic names as the cause of a leaking seal