How to Replace an RV Toilet: What Fits, What to Measure, and the Swap Step by Step

Can you fit any RV toilet?

Any gravity flush RV toilet whose clearance figures suit the space will go in, so measure before you choose a brand. Dometic's 300, 310 and 320 installation manual is written for replacing the existing toilet; a new location calls for a qualified service professional.

Check three things on the toilet you are removing:

• The water line comes through the floor or the wall. From the floor, you need an elbow adapter kit.
• It has two bolts perpendicular to the back wall. If there are four, you need Dometic's Universal Mounting Kit.
• The flange is level with the floor. If it sits in the sub-floor, you may need a taller seal.

What to measure so the new one fits

Write down these six figures before you order.

1. Floor flange to wall: the rough-in figure the makers publish for each model.
2. Flange inside diameter and thickness. Dometic's 300, 310 and 320 need a 3 in. (76 mm) flange, 0.25 to 0.44 in. thick.
3. Flange height. If it sits too low, Thetford's spacers bring it to 7/16 in. above the floor.
4. Where the water line comes out, and its size. Dometic's inlet is 0.5 in. NPT; Thetford's Permanent RV Toilet manual calls for a 3/8 in. supply line.
5. Side wall space and door swing, with the lid up.
6. Pedal height: Dometic wants the pressed pedal to touch the floor the toilet stands on.

From Dometic's 300, 310 and 320 installation manual (rough-in included, may vary by 0.38 in.) and Thetford's Permanent RV Toilet manual (minimum distances). Check the drawing in the manual before you cut or move anything.

Water flow decides whether the flush works: Dometic rates the inlet for 30 to 100 psi and wants 3 to 5 gpm, with 2 gpm the minimum and a flow restrictor above 5 gpm.

How to swap it, step by step

Empty the holding tank first, and wear gloves, eye protection and a face mask as Thetford's kit sheets advise. You need pliers, an adjustable or torque wrench, a 1/2 in. ratchet or box wrench, a putty knife and a small bucket.

1. Turn off the water, flush to drain the line and put a towel under the water connection.
2. Disconnect and drain the supply line.
3. Remove the caps, nuts and washers, lift the old toilet onto a trash bag and discard the old T-bolts.
4. Cover the tank opening with a towel, then scrape all old seal material off the floor and flange.
5. Check the flange. A damaged or glued-down flange has to be replaced, and Dometic suggests a qualified service professional for that.
6. Fit the new floor seal to the base of the new toilet, lip side down (some models arrive with it fitted).
7. Set new T-bolts in the flange slots, lower the toilet onto them and press it down with your body weight, so the whole outlet sits inside the flange hole.
8. Tighten the nuts alternately side to side so the base does not distort: Dometic's figure is 30 to 40 in. lb; Thetford says tighten until stable and do not overtighten. Press the caps on.
9. Connect the water line with 1/2 in. NPT fittings at 30 to 40 in. lb, with a 1/2 in. elbow adapter first if the line comes from the floor.
10. Turn the water on, flush several times and check every joint. Then fill the bowl and leave it an hour, which is Dometic's leak test.

Can you put a household toilet in?

No. No toilet maker allows it, because the plumbing is different. An RV toilet drops waste straight into the tank below, and Dometic's gravity discharge manual states that horizontal pipe runs as found in conventional residential plumbing cannot be accommodated, which is exactly what a household trap needs.

Water use is the second problem: Thetford puts an RV flush at 1 qt against 1.6 gallons for a home toilet, which would fill the black tank fast. Keystone, Winnebago and Jayco all refer owners to the toilet maker's manual.

Porcelain bowls and taller toilets

You can get a porcelain bowl and more height from RV models. Dometic's 310, 311, 320 and 321 and Thetford's Aqua-Magic Style II have ceramic bowls, so check the RV toilets sold by spec. For height alone, Thetford's Toilet Riser adds 2.5 inches to any Aqua-Magic or the Bravura.

Switching to a macerating or composting toilet

A macerating toilet adds wiring: Thetford's Tecma 2G manual wants its own breaker or fuse and a 1.5 in. discharge line, and allows up to a 6 ft rise to the tank; Thetford recommends a licensed tradesperson for the electrical work, and Dometic a qualified RV technician or electrician. A composting toilet drops the black tank and water line, and adds a vent fan and a solids bin.

A leak at the base after the swap

A leak at the base usually means loose mounting bolts or a bad floor seal: tighten the bolts first, and if it still leaks, lift the toilet and fit a new seal, or check the flange height with Thetford's spacers. Other faults are covered in RV toilet faults.

Common questions