May not be the blank tank. I suspect you're smelling the gray tank which can be as bad as the black tank. Be sure all of the traps have water in them ....including the washer drain. If this does not help, then start replacing the air admittance valves under the sinks ($5.00 approx at Home Depot).
I don't have a washer, so I've taped over the drain line so I don't have to worry about the trap losing water.
Also be sure you don't have a stove burner slightly turned on as propane can smell bad also.
Outgassing on a bad battery can be bad also.
Good luck.
Fred