Have to tell a FUNNY.
We had the same problem with the rubber seal leaking. I removed the seal in an attempt to locate a replacement at a local RV parts store, but they didn't have it. Decided to clean really well, reinstall and add hot water, as suggested, somewhere on this forum. Well, I poured a 2.5 gal jug of hot water in the toilet after reinstalling the rubber seal, and it immediately began to leak. I stuck my hand into the water to press the seal a bit, and helped it seat. I watched the water for a bit, and it didn't seem like it was going down. YAY! I closed the lid, saying to myself, I'll check it again, later. (That was last weekend.)
Well, Saturday, DH decided to haul the rig over to get it inspected. After he returned, I went in and discovered water all over the bathroom floor! I panicked at first, trying to figure out where the leak might be, but how could it, we were not hooked to any water source??!? Then I remembered, the 2.5 gal of water in the bowl, probably sloshed around and spilled out of the bowl as he drove the rough roads around town.
At least it was a clean bowl and clean water! And there was still a lot in the bowl, so the seal is working!
E