All of the rigs I know about have 4 tanks. One fresh water, 2 gray water, and 1 black tank. There is the possibility yours may have 2 fresh water, but I doubt it. Usually the only time you'll use your fresh water tank, is if you are camping someplace that doesn't have fresh water, then you'll want to fill your fresh water tank, we'll fill it at the dump station when we arrive at the campground, with the hydrant marked POTABLE WATER. I also carry a spray bottle with a Clorox mix to spray it with, you never know what some idiot may have done! If you are camping someplace that has a water hook up at your campsite, you can hook up your hose to their hydrant, and the other end to your "city water" connection, and you'll have water in your rig just like at home. We don't travel with water in our fresh water tank, as it adds a lot of weight to the coach. Sometimes I might put some water in it for restroom breaks while on the road, but usually just use one at a rest area, or wherever. If we are conservative with the water, and have a full tank, we can go for about a week before having to refill the tank, or dump the tanks.