'P' traps holding water?
Toilet bowl seal holding water?
Toilet floor flange good...toilet tight to floor?
Tank vent lines clear/unobstructed?
Air Admittance Valves functioning properly?
Sewer fittings tight with good o-rings?
Then no smell in rig.
Something has to be broken in order for stink from tank to get into rig. And I'm not talking about those whiffs when flushing toilet and exhaust fan on or those whiffs that come from roof vents thru open windows when traveling.
Dump black tank when full (or close to full)
Rinse/flush black tank get a clear sewer adapter so you can see)
Add few gallons of water back into black tank after dumping/rinsing bottom stays wet, stuff doesn't stick/dry out)
Fill toilet bowl full and flush poop
Only chemical needed is.....water