My '03 Fleetwood TC has about a 33 gallon fresh water tank. The black and gray are about 22 gallons each I think. This works for our family for a 2 night stay just fine. The gray tank will hold about 4 showers plus other hand/dish washing. I think I could probably go 5 days or more on the black tank but I've never tried. Fresh would probably last us 5 days too.
By the way, I owned a Palomino truck camper once; I won't own another one. The construction and materials were very poor. Some people have said that a Fleetwood camper isn't that great but mine is miles ahead of the Palomino I owned.