Interesting topic. I can see both sides of the issue but most places clearly say one "camping unit" per site.
The problem is there are plenty of gray areas, such as:
1. A TC pulling a cargo trailer containing a bunk or maybe more.
2. A pickup with canopy with a foam pad, pulling a TT.
3. 2 sedans with 2 tents set up.
For whatever reason, all 3 are likely considered 1 camping unit despite being ever so similar to a TC pulling a TT or 2 TC's.
Arguably #3 is the highest impact to the facilities with 2 tents, outdoor cooking, etc. but that isn't what they charge for. If they are really charging for the space, why should it matter what type of equipment occupies the space?
It seems to me like the fairest solution should allow whatever vehicles you can fit in the spot, while also remaining under the limit on number of people per spot. I'm even ok with charging for each vehicle since more vehicles = more traffic on the camp roads. When you also consider how full campgrounds are these days, why not allow for more people to enjoy, provided the site maximum occupancy is met?