I just don't worry about it. When I call to see if there is a space available, I state that we have a 32 foot motorhome with a towed vehicle. It is up to the management to decide if they can accommodate me. If they say they can't, I simply go somewhere else.
When we go to Forest Service campgrounds, where there is no person to call and there are no reservations anyway, we simply check the available sites (if any) and choose the one that looks like it will fit our needs. I have put a 29 foot class C with a towed Jeep in a 25 foot Forest service site. It worked just fine!