I called a couple of campground managers, as well as tech support for the reservation websites about this question in the past. They told me that the max length on the reservation or campground info factors in a generic tow vehicle length, which is why the Class A/C length is longer than towables. "Just use in your RV (class A/B/C/5er/bumber pull) length and don't worry about the tow vehicle."
Some places, in my experience, under-estimate the TV length, but I've been able to park the truck crosswise in front of the 5er and stay (barely) off the street.
