If the washer drain piping has a trap it may be void of water; trap water would keep the graywater tank odors from coming up the pipe and into the coach. If the trap and drain aren't properly vented, then each time you dump your graywater tank it creates a vacuum that can suck out the trap's water.
Check that air can get in either at the point the washer's drain hose connects to the drain pipe, and/or that any trap in the washer drain pipe system has a special vent valve above it. To readily see what such a vent should look like, just take a gander at the trap setup under your kitchen or bathroom sinks where they are easy to spot. At least run some water thru the machine or pour a few cups down the drain so the trap is full. That should plug off odors emenating from the gray tank.
Just driving down the road can slosh water out of a trap, and if not used they can just plain dry out, so if you infrequently use the washer you need to occasionally run a short cycle to fill the trap. Our shower's trap isn't vented, and often gets stinky immediately after I dump tanks, because the vacuum caused from dumping sucks trap water down to the emptying tank; a cup of water down the shower drain refills the trap and solves the problem.
Sometimes front-loader machines retain moisture around the door seal, and it can eventually grow mildew or mold. Make sure you check deep in the seal and wipe it dry if needed after loads, esp. if you don't use the dryer cycle.
Hope this helps.
Joel
