Toilet bowl valve does not seal

[Phil Saran]

I have a 2019 Cougar 30 RLS 5th wheel, we bought it last October
used it for the month of November and then winterized it.

Brought it out of storage and de-winterized it, but the sliding valve
in the toilet does not hold water.

It the correct solution to either spray the sliding valve with WD40
or apply some Vaseline to the sliding valve or something else??

[JRTJH]

I assume you have a Dometic 300/310/320 toilet???

If so, the correct procedure is to use your finger, reach inside the toilet with the foot valve depressed, pull the seal up toward the bowl to "massage it into position". If that doesn't work, then apply a SMALL amount of oil (I use olive oil) to the seal, pour a half gallon of "hot (not boiling) water" into the toilet and allow it to sit until cool. That will "heat mold" the seal. If neither of those will work, then buy a replacement seal, remove the old one, CLEAN the toilet thoroughly and install the new seal. Repeat the "massage and/or hot water molding" as needed. The procedure is in your Dometic toilet owner's manual.

It sounds like your toilet may have frozen during the winter. Look at the back surface of the flush valve, and see if the "freeze indicator" is protruding. If so, you'll also need to replace the flush valve. There are photos of the freeze indicator in the owner's manual as well.

If you don't have/can't find the owner's manual, you can download it from the internet, it's only a 4 page document.