If a ball/check valve were a good idea, don't you think at least ONE of the toilet mfrs somewhere in the world would already be doing it??? Yet you won't find a ball valve in a single one, not even in megayacht toilets. You're ready to reinvent marine toilet technology and you don't even know the model name of your own toilet and can't even be bothered to get out the owners manual for it!
And whoever designed a system (I suspect it was the builder...builders are the worst sanitation system designers on the planet!) that requires a 26' run to the tank starting with 6' lift was even more clueless...'cuz if your toilets are SeaEras as I suspect, they can only lift bowl contents 4' or move it about 6 linear feet in a single max flush without a LOT of help from gravity. Your problem isn't that you need something that you consider better than a joker valve....both toilets should never have been connected to the same tank! The only
improvement you should be considering is a second tank just for that toilet that's a LOT closer to it with a plumbing run that can let bowl contents actually get there!
And that is all I have to contribute to this thread.
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein
Specializing in marine sanitation since 1987
Author "The NEW Get Rid of Boat Odors -
A Guide To Marine Sanitation Systems and Other Sources of Aggravation and Odor"