Seepin' Toilet Seal

I have a question about RV toilets. The one in our older motorhome seems to work fine but I'm wondering if the valve is closing properly. My neighbor says that the toilet should hold water in the bottom bowl to make a seal to prevent odors. All the water leaks out of mine and into the holding tank. I cannot find anything that may be preventing it from closing completely. Is this a do-it-yourself repair or do I need a professional?
Harv, (La Crosse, WI)

Harv, indeed, there should be some water left in the bowl after every flush. Without the water seal, odors from the holding tank can and will enter the coach. There are a few possible reasons for the water to leak out; first, check to be sure that the bolts which secure the toilet to the flange are not too tight, thereby distorting the base of the toilet. If the toilet is mounted on top of carpeting, trimming the carpet so the base rests solidly on the floor is best. If the base is too tight, the flushing mechanism will become warped or wracked and not close fully.

But the most common cause is simply that paper and waste have accumulated inside the slide valve mechanism on Thetford’s Aqua Magic units. On an older SeaLand toilet, the typical cause is a worn seal or a clamp ring that is too loose. Yours is likely to be one of these two brands.

If you have an Aqua Magic, fashion an L-shaped hook out of a coat hanger or a bent screwdriver that will reach into the groove to scrape out the residue. Turn off the water supply, depress the flushing pedal and carefully remove any paper jammed into the groove just below the rubber seal. There is a tool you can purchase from Thetford just for this task if you wish. Take special care not to damage that seal. In severe cases, the toilet will have to be disassembled and the flushing mechanism taken completely apart, then cleaned, lubed and reassembled.

To rectify the situation on the SeaLand toilet, simply tighten the clamp ring. In some cases, a complete seal kit may have to be installed if water still continues to seep past the seal and into the holding tank.