We travel with at least 1/3 of a tank of fresh water - for the purposes mentioned. If we're going to stay someplace that doesn't have sewer connections, we'll finish filling it up before we pull in and set up and run off the tank/pump to measure and conserve what goes into the gray tanks. About 4 - 5 times a year, we put 1/4 tank of clean water with Borax crystals and dish detergent in each of the waste tanks before hitting the road. This is called the "Geo Method" of cleaning the waste tanks. We've not had any issue with sagging tanks, though - even when the fresh tank is full.
Rob
