Toilet to holding tank seal

[Free Bird]

I have a Forest River Georgetown Class A and thought I had a major problem but it turned into a minor one. However, The seal from the toilet down pipe to holding tank was leaking at holding tank grommet. It is soft rubber and has a flange which goes into tank and other half remain on top of tank. Several examples of useless sealer were there, but none were working. MY QUESTION IS,: what do I use to seal this rubber grommet to holding tank. Whatever was in there was black, but apparently useless, did not stick to anything, rubber or tank.
Free Bird in SW Florida

P.S. The greatest stuff I have ever seen, a holdover from my days as a wooden boat builder, is a material that we used to call bedding compound. it was tan color and very sticky and never hardened. Used for attaching deck hardware, and it never failed. Has anyone ever tried this?

[Free Bird]

Cancell above request, just finished total redo. Pulled toilet, down pipe, tank grommet, and the problem was apperant. None of factory sealer was working. Broke loose and fell into holding tank in big pieces. Was leaking at grommet when near full. Bought new grommet, and toilet base seal, and cleaned everything up. ( UGH), However, it had it's bright side also. Was able to reach thru and cleaned interior of holding tank and now even the tank level sensors work. Even rebuilt dump valves. We are good as new. By the way, used ProFlex RV sealant and seems to work great.
Thanks