There are a few other thing s to suggest. Is the vent pipe 2" or more above the roof... if not an extension could be added to make it higher, since it won't vent right if it is not high enough..Also, some RV manufacturers make the vent hole down to the holding tank bigger than it needs to be--- so check and make sure all is sealed and tight around the vent pipe.
Also, the vent pipe sometimes falls down lower and is sitting IN the black or grey water holding tank, therefore not venting.
Winds can blow down the vent pipe causing odors in the RV. There is a top for that , that moves directionally with the wind to eliminate that problem. Think I sent you a photo and website of that recently via e-mail.
The P trap in nearby shower can drain dry or some showers have been known not to even have a p trap, and can suck the water from the p trap of the sink. To test this out, tape plastic over the shower drain and see where the odor is coming from then.
Also, hair and other things can lodge in the sink drain pipe between the stopper on the sink and the P trap of the sink. One can take the stopper apart and clean out that sink drain pipe. Gunky hair and trash can get really smelly, even with the P trap full of water as it should be.
The longest we've had a RV is five years...but we have never had to replace a commode seal, so makes no sense you'd be soon replacing yours for the third time. Is there some sort of "buldge"... rough place, uneven floor place that could cause the seal to not be making a good connection all the way around. I'm talking the seal on the floor now.
The flapper seal can get rough sandpapery residue on it..and doesn't allow it to seal good. Mostly caused by hard water, as the others said. We have that trouble in the workshop commode, but not in the house where we have "conditioned water"...the workshop uses our "original water" and leaves hard water residue and stains. If you used your stone to clean that hard residue off the porcelain commode. You could wet paper towels and mop up the grainy residue droppings with the paper towel...cleaning it as if there were NO drain...and therefore not letting that grit wash down into the holding tank-- you mentioned you didn't wish to do that.
Trying to make suggestions not posted thus far. Hope you find a fix....and no longer have problems!
Happy RVing!
Joella