Thetford toilet water kept running

Sometimes ours would leak and fillup the bowl all I ever did was depress the flush pedal and release it by slipping your foot off while fully depressed. Often took several times to free whatever crud, lime buildup, was keeping the valve from sealing.

The pedal is wearing/binding on the plastic shaft on which it rides. This can be
checked by lifting the front of the pedal with your toe.
If it stops seeping, you found your problem. It. Will be an intermittent problem until
either the pedal flies off or the bowl overflows.
The inexpensive fix is to buy your Dometic 310 and replace promptly, or keep chatting
with Thetford techs who claim to be unaware of any problems and recommend you buy
their rebuild kit for $40.
This is a common problem if you use the search or Google the problem.
Hopefully you'll get this solved before the toilet overflows.

There are reported issues with that model Thetford on just about every RV forum you read. If the pedal comes off a replacement from them is approx 50 bucks. So the common cure is either the Dometic 310 or 320 depending on the space you have.