flush valve seal in Thetford AM II

My RV was purchased new in 2017. The bowl held water well for a few years, then started leaking down past the ball valve. So far I've replaced the seal twice (with liberal amounts of plumbers grease), ball valve once and drive arm once. I cleaned all areas as best I could whenever it was apart. From what I've read, it's a real pain to change the spring if you can find one. I'd rather not buy a new toilet. Has anyone tried putting butyl tape inside the seal or on either side of the seal to fix the problem?