Just use a decent amount of water each time you flush and don't dump until your tank is full. We have been full-time for 6 years and have had no problems. We use any septic safe 2 ply toilet paper, just don't go overboard with it. No chemical, just water and we rinse tank maybe once a month.
Oh , and the level indicators for the tanks never work for more than a few weeks, at least not with accuracy. You will learn when you need to dump as the black waste tank with start making belching, gurgling noises as it near full and the gray will back up into the shower when it's overfull. We just know when based on the number of showers, etc when it's about time.