Ok, so if we take a step back, I see two issues that need to be addressed:
1) Smell from the grey tank is making it back into the trailer.
2) There's a smell coming from the grey tank.
1) Examine all air admittance valves and traps. Except for a hole in the plumbing that's the only way smell can make it back into the trailer. As others have said, a valve could be faulty, or a trap might not be holding enough water, etc... Repairing whichever one is at fault is the only way to address those.
2) We need to determine why there's a smell in the first place. Either it's coming from the campground's lines when hooked up to sewer, or it's coming from something in your tank. To address the campground, either don't hook up, or at least make sure your dump valves stay closed. If it's coming from your tank, than you'll want to clean the tanks. More than just a straight fill and dump, you might also need to put in some kind of cleaner, either some TST, the Geo Method or similar, and then drive around to get it to coat the tank and give it a good cleaning.
I suspect it's the second case and your tanks need a good cleaning. Especially since you said your monitor always reads 1/3. That's often indicative of sludge build up.
There's no use crying over spilled milk, unless it's on your keyboard.