Horrible smell from RV toilet - any recommendations?

I used to get smells from my grey tank. About once a year it would happen. First I figured traps were dry but that wasn't it. Had a friend tell me he had same issue and the fix was to flush down the roof vent. So about once a year I'd pull the hose up on the roof and blast some water down the two vent pipes. It fixed the problem but still don't know why. I never looked under the sink for an air admittance valve and there should not have been one since there were vents. Just sold that one.