You all must be staying in fancier motels than I do, or visiting more popular places than I've visited, or something. I think I've only been on one or two trips where I can say with certainty that camping in my class C was less expensive than driving my car and staying in a motel. The accommodations are often less expensive (sometimes only a little less), and the food if one cooks for oneself, but the fuel is considerably more.
A good bit of the time, there aren't any motels or hotels to be had where I've camped, so it's hard to make too close of a comparison then.