Originally Posted by bedrck46
You may also want to get up on the roof and check the vent pipes to make sure they are not clogged up
^^ My guess too. If this is happening when you are dumping the tank, I would bet that there is not enough air getting down the vent stack, and the suction is pulling the water out of your p-trap(s) allowing fumes back up into the coach. Don't just mask the problem by trying to cover it up; make sure that the underlying problem is solved as well.