As several posters have made clear, if you try hard enough, you can make a logical argument to justify just about anything. The biggest argument for one rig per site is simple, that is the rule and it is easy to be enforced. Start letting two TC units, soon two "small" class C rigs will want to occupy one site. Then two slightly larger class Cs will argue they are really no different than the smaller one, and on it will go. Then it will not only be when the park is full, but when it isn't, so now revenues are decreasing.
Arguing two small TCs have less impact than one large RV is about as effective as arguing two thin people would have less impact on the all you can eat buffet than one fat person. It may very well be a true argument, but it isn't one to build a policy on.