In addition to switching to the geo method and checking tank vents, you really need to be flushing your tank. I know it's a pain and it's not fun but it really helps clean the nastiness out. That is probably the biggest source of your problem. If you don't have a built in black tank flush then get a wand (they have them at Wal-Mart) and a hose (not your fresh water hose) and close and rinse the tank a few times each time you dump. We get a lot of stuff out this way every time.
Additionally make sure you do not leave your black tank open when on full hook ups. Always keep at least a few gallons of water in the black tank after dumping. Failure to do so results in building the dreaded stone mountain and that can smell bad. Make sure you flush your p traps because if they dry out, they can allow odors to sleep up. Also do not flush toilet with the bathroom fan running as it can suck the odors out of your black tank and into your trailer.