First check 'all' your p-traps and made sure they are filled with water.
If you let your RV sit for lengthy times the water evaporates in the p-traps. Make sure all sink drains AND the shower drain are filled.
The usual smelly culprit is the bathroom sink that usually dumps into your black tank that can emit some really nasty odors if the p-trap is not filled.
Gray tanks can smell as bad as a black tank.
Dump the gray and add a bottle of lemon cleaner fill with water and dump again.
And of course make sure there is water in the toilet bowl at all times.