I'd check the 'cheater vents' under the sink cabinets. These are used to vent P traps that aren't connected to a rooftop vent. Also known as an Air Admittance Valve, they can fail and allow sewer gases into the under cabinet area. Easy to replace, they just unscrew and get replaced. Found at Home Depot, Lowes, etc.
Also make sure the toilet valve is closed and allow a little water to sit in the bowl to insure the valve is closed and not leaking.
Next time you dump the black tank, you might string a hose into the bathroom and run water down the toilet to help clean out the black tank. Also make sure all traps have water in them. The shower and if you have washing machine plumbed, make sure that trap has water in it.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI