Guess driving stirs it up. The weird thing is i always have water in the bowl but the smell is still really bad?
So the smell isn't bad until you drive, then it's bad for the entire trip? Sounds like you either have a poop mountain in the tank or your vent black tank vent has cracked, poop mountain fix is 20 or so lbs of ice in the tank and alot of quick turns, start and stops for about an hour of driving, put in a few gallons of water half way through to help, but ice only first.
You say you flush the tank sometimes, is that with a wand (through the toilet drain) or quickie/tornado flush device (hose attached from the outside)? Reason I ask is "if" you overfill with the quicky or tornado you (or the previous owners) could have burst the vent line, causing an air vent into the trailer.
Either way good luck. We used to have smell in our trailer and after the tornado flush I've never had it again. It only took one good cleaning of the tank to eliminate the smell. I now "try" to clean the tank every 2 or 3 trips but don't worry about it untill the end of the year so I can start fresh in the spring.
Good luck.