1) Crack in vent pipe allowing venting into the cabin. Time for a flashlight and careful inspection. If you find a crack or a bad joint (e.g. not glued), fix it.
2) Proper venting through the roof, but the sewer gas is finding its way back in through windows or other vents...e.g. the roof vent. Compared to the factory roof vent, there are some turbo charged options. Illustration/not recommendation: https://smile.amazon.com/Lippert-Com...9594103&sr=8-1
3) Since this seems to be unique to the full-hookup campground, a LIKELY cause is some pressurization of the sewer drain network when other RVs flush or dump...especially if you drain continuously rather than allow the tank to fill then dump at once. Consider a dump from a black tank containing 40 gallons or more of effluent...rushing down the pipes all at once. In a poorly designed RV park drain system, every dump may push sewer gasses out the other connection points. The other connection points are serving as vents. I've notice this when we were in a park that had connections, but we were in a pop-up that had no black tank. It was pretty dreadful to be near our connection point when someone else dumped. This pressurization may exit your roof properly, but the sheer volume of stinky air may create a stink cloud around your rig. But then again, the mistake is yours. Do NOT drain continuously. Let your black tank fill...with the dump valve closed...and when it's about 2/3 full, let her rip. Sewer gasses won't penetrate your black tank dump valve if it's closed, and solids won't build up in your tank as much since they are "flooded" most of the time.
4. "Dry trap syndrome" at your washer/dryer hookup. The grey tank in your rig is also very stinky. If you have a dry trap there (or anywhere), you'd swear that the grey tank smells as bad as the black tank. Routinely add water to ALL traps after travel and dumps. Each and every dump can cause the water in any given trap to get sucked out, and jostling down the road can cause the water to trickle out of the trap to the point where their is an air gap for stench to penetrate the cabin. Also bear in mind that a "hermetically sealed" RV cabin can be slightly pressurized with the opening and closing of doors. That pressure can push a bit of water out of a trap. Just last week, I dumped my rig, and as is my practice, I use a garden hose to jet water down all sink/bathtub drains while the grey tank valve is open. In one case, the violent rush of water from the hose did the job (cleaned the traps), but it also caused one of them to over-empty and be "dry." The camper stunk like hell until I ran water in all the traps, and voila, fresh air.
5. On the theme of dry traps, grey water drain plumbing in most RVs relies on what's commonly called an "insta-vent." https://smile.amazon.com/2-inch-Tuub...NsaWNrPXRydWU=
Like everything on an RV built to price, they wear out. It's easy to replace them.
6. If you're anywhere near a toilet, the gaskets on the flush valves have a way of wearing out. Test? Fill the bowl with an inch of water, leave, come back in an hour. Is the water still there? If not, the way the water leaked out is the way sewer gas can leak in.
I think I covered the bases. But whatever it is is a pretty simple fix.