Not sure if you have a washer/dryer installed, or if the connection might be un-capped. This will occur from periods of non-use, or when traveling. It could be that your Washer connection P-Trap has dried out. A P-Trap uses a small amount of water to block the rest of the plumbing system from venting indoors. Your W/D drain connection wouldn't be tied to your black water tank normally, but it is tied to a grey water tank that can stink worse. If your grey water gate valves are open, it would essentially be open to the entire campground sewer system.
You might want to pour a cup of bleach and water mixture in the drains, including the washer connection, after traveling or periods of non-use. Also, your vent (exhaust) fans can pull smells in in through that path.