I typically carry between 1/2 to full tank of fresh water depending on the destination and length of the trip. In the case of my coach I can carry about 86 gallons of fresh water, which will weigh about 720 pounds, my coach has a GVWR of 17,000 pounds, and when loaded for travel it typically weighs in around 16,500 (including 2/3 full tank of fresh water). I would not consider traveling with no fresh water, at a minimum I would want 30 gallons, in case of unexpected events (getting stuck overnight due to mechanical issues, etc). So at best we are talking abut a 500 pound difference in total operating weight (or about 3% of the total weight of the coach), which I doubt would effect fuel economy significantly.