Black tank, wait for it to start burping(air bubble, it's almost time to dump. Grey tank, I have 2, the bathroom tank fills fastest and I have been known to be standing in grey water while showering. Empty immediately. The kitchen grey tank takes longer to fill but I usually empty it when I dump the black tank, in tandem with the bathroom grey tank. It seems the gauge for our fresh water works great. When boondocking, we can usually last about 10 days without having to dump the black water.That's me having a navy shower every day. I have two water bladders, one is a 50 gallon and the other is 45 gallons. When the 5er is dropped I go and fill both of them and have water ready to go when needed. I use a RV pump to pump the water into my gravity fill port. My dually has an 8 ft box so both bladders can be used at the same time.