Hi Roo
Since the tanks are holing tanks and not septic tanks the cleaner you keep them the better.
For the back tank I pour two caps of Calgon liquid water softener in the toilet. It will make the tank walls slippery. Then I travel with about just less than 1/3 tank of water. Let it slosh around while driving. At camp I let it fill up before draining. If not full before draining I fill it. Get a good flow of water when emptying. I also use some little blue packet if deodreizer with each tank.
After emptying the tank I fill it again. Just water. Fill it full. When you empty it the second time you will be surprised how dirty it comes out. I suggest getting a clear drain tube connector. It connects between your sewage tube and your camper connection. It enables you to see how the water is as it drains.
When I prepare my camper for long-term storage I fill it a 3rd time and pour 1/2 galln bleech down the toilet. Fill the tank let it sit 30 mins and drain.
Do this and you should be good.