I've only been doing it a short time, but I can attest after our last trip to the Calgon and Dawn treatment for the black tank. The grey tank should be fairly clean since you put soap in it when you wash your hands or take a shower. Unless you're throwing solids like food scraps (which you shouldn't be doing) in the sinks there shouldn't be any. As far as the black tank, when I dump it, I immediately mix a cup of Calgon and a cup of non-concentrated Dawn in a gallon jug of warm water and pour it down the toilet. So far so good....the tank seems to flush out very well at the end of the trip and no smell!! We also pour the soapy dish washing water down the toilet....(we wash in a pan in the kitchen sink instead of just rinsing it down the grey tank.)