One of the nice things about having a big bus is the capability to carry lots of water. A class C motorhome probably will only have a 30-50 gallon fresh water tank. I had my tanks made since I have a transit and had to use every inch I had. I ended up with a 130gal fresh, 85gal gray, 45gal black. Now on my truck conversion, I'm going even bigger (because I can) to 210gal fresh, 110gal gray and 65gal black. Having small tanks is really a handicap to being able to sit for a week dry camping and not having to worry about water. I found that if you figure about 10gal per person per day of water, then you can size your tanks (our current 130gal tanks will get the two of us out to 7 days). Good Luck, TomC