You might also consider Calgon Water Softener (use the liquid, not the powdered) in about three or four gallons of water per tank. The "soft" water will swish around (how's that for a technical term?) during travel and will help keep "things" from sticking to the sensors and the walls of the tanks or valves. We've been doing this for years now (after hearing about it here on the forum) and haven't had any problems with our tanks. We add the solution each time we dump our holding tanks. Therefore, our tanks are never dry. Also, every once in awhile, we'll do the add-a-bag-of-ice trick before heading out on a trip, just for good measure.