OP update - fixed my problem. I think the underlying problem was caused by a failing ball valve seal. The water only vs flush issue started last season and thought I had it fixed with a little lube and some relief on the back of the toilet so the rod did not bind up.
De-winterized a week or so ago and this year the toilet would not hold water along with the issue of flushing immediately upon depressing the flush pedal. Talked to Thetford and ordered both a new ball seal and a new water control valve. Replaced both parts tonight and toilet is working like new.
I think the real issue was the ball seal. While playing with the unit to see what the problem might be, I could put just a bit of pressure on the arm that does the water flow and the operation was normal (rod slides down a slot in this lever to add water and when rod reaches the end of the slot, opens the ball valve to flush). I think the new ball valve seal added enough pressure on the ball valve that the rod slid down the slot and worked as designed. Similar to what I did with my finger on the lever arm, the extra sealing tension of the seal to the ball valve allowed the rod to go down the slot, add water as designed, and then open the ball valve when the rod reached the end of the slot. And now it holds water just fine.
So, my suggestion, if your foot flush valve is getting finicky about just adding water, first I would change the ball valve seal and lube the rod while you have it all apart. The extra tension of the new seal on the ball valve may be just enough to cause the "add water" function to return to normal.
Tom & Mary plus Lilli the Havanese
2017 Entegra Aspire 44B,
450 Cummins, Spartan K2
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