Check to see if the tank vent line is plugged--opening the valve is letting odors escape that have built up--a vent should be handling that. Up on the roof, locate the vent cap and take it off--usually is clipped on, but may have one screw holding it down. Should have the black poly line sticking up thru the roof line--if not, it has slipped downward and it sitting on the bottom of the tank. If it is there, then check that it is open down to the tank.
Outside of the above--do you happen to have an exhaust vent in the bathroom? If it is on when the toilet is flushed, it could be pulling odors from the tank.
'05 36TK3 Mobile Suites
'10 F450 new to me
'09 F450 Lariat 4x4--died