If there are any P-traps that have dried out (washer plumbing but no washer, etc) you may have gases passing back into the trailer even after one use of the black/grey tanks. Another possibility in a used trailer is the anti siphon vents under a non roof vented sink P-trap may be faulty. They look like this.
Towr: 2007 Country Coach Allure 470 - 37 Sunset Bay Cummins 400 ISL
Toad: 2016 F150 King Ranch - 3.5L EcoBoost 4x4 Supercrew (curb weight 4,775 lbs)
Toad: SOLD 2007 Jeep Grand Cherokee (yes, it has a Hemi) (curb weight 4,720 lbs)