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
2017 Ford F250 Lariat, 6.2 gas
Equalizer Hitch w/ 4-Point Sway Control

Days camped since 2016(retirement): 920