Thetford toilet not holding water and smells

I’ve got a thetford. Start by cleaning the ball seal. You can pour a little hot water (not boiling) into the toilet and open and closing the flush ball to remove any debris that might be preventing it from sealing correctly. You may even have to put on a rubber glove and use your finger to remove any build-up around the seal.

If that doesn’t work, you have to replace the seal, which is fairly easy (at least on my model). In a nutshell:

1. Turn off the water supply to avoid leaks.
2. Loosen the two nuts at the back of the toilet to separate the bowl from the base.
3. Once removed, the seal can be accessed and replaced. Clean the area if needed and lubricate the new seal with silicone spray.
4. Reassemble the toilet by reversing the disassembly steps: lock the bowl back in place and secure it with the two bolts.

You’ll want to check your toilet manual to make sure the process is the same for your model.

Great info by @billgreene above. I would also recommend using Thetford Toilet treatment at least at the start and end of camping season: https://www.amazon.com/Thetford-Toilet-Lubricant-Conditioner-24-36663/dp/B002NSU0ZW/. A little maintenance goes along way.