It depends on your RV, I have shallow tanks and therefore use plenty of water. Otherwise you end up with a solid puddle directly under the toilet. My parents borrowed my coach sometime ago, and being of a previous generation, thought they needed to conserve on everything. Needless to say, I had to fill the black tank with water and then drive a few hundred miles before the tank would empty. So the toilet is one area that I do not conserve water. I only have a 23 gallon black tank, but have only filled it once in 12 years. I usually try to leave around 3 inches of water all the time, and let it rinse very well. Its just easier that way.
'83 Revcon Prince 31' FWD FMCA F298817
502 w/Howell/GM ECM/Edelbrock MPFI,Thorley's & Magnaflows, 4L85E 4 spd, KoniFSD