Unlike a class-A, the black tank is easily cleaned with one of those spray wands down the toilet that is designed for that purpose. The toilet sits directly on top of the black tank which is above floor level. So all one has to do is use the cleaning wand and spray all around until the tank is about 1/4 full. Then empty it. I find that doing this process twice is enough to have the drain water run fairly clear.
Because the tank is above the floor, and the outside drain is right below it, the contents flush out pretty quickly. With that good flow, not much stays behind.
The grey tank however is another matter. It is below the floor and the drain is almost level with the bottom of the tank so the flow is more leisurely. When I want to clean out the grey tank, I will run the passenger wheels up on blocks to tilt the trailer toward the drain. This is not needed for normal camping operations but a couple of times a year I like to give it a good flush.