In our old camper we had the same problem.
Removed the toilet and put a regular household "wax ring" on the flange, reinstalled the toilet, and never had another issue.
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Craig & Cath
2018 2902WS Rockwood Ultra Lite (with tons of Mods)
2022 F250 Lariat Super Cab, 7.3 gas w/10 Spd Xmsn
Equalizer Hitch w/4-Point Sway Control
Days camped since 2015(retirement): 1687