I, too, have the MaxAire vent covers and keep my roof vents open all hear round. And I keep chemicals in my gray tank and black tank throughout the camping season. As soon as I dump and flush the tanks, I add the Thetford's chemical and about 3 gallons of water. Also make sure there is water above the ball valve in the toilet and in the P-traps of the kitchen and bathroom sinks.
I had my previous TT for 9 seasons and never had any smelly odor problems, following this regimen. But I have been in used trailers on a couple dealer's lots that stunk to high heaven! So I know what you mean.
2015 Jay Flight 32RLDS Elite
2012 Chevy 2500HD Crew Cab LTZ (6.6L Duramax/Allison)
Equalizer Hitch (1200# bars)