Different campgrounds use slightly different definitions, but overall, I belive it means:
The Site Length is the length of the spot you are using. Basically, it's the private use spot assigned to you.
Driveway Length is the length of the driveway from the campground road to your actual camping site. You may be the only site on that driveway or it could be a shared driveway with another site. The driveway may not be level or straight. Generally the driveway is for access to the site but not part of your private site.
Maximum Vehicle Length can be the maximum length of the combined RV set up or the maximum length of any single piece that the campground sets based on what can negotiate the campground, the driveway or the actual spot. It may be because of a turn or narrow roads, or tight squeeze, etc. You need to verify with the campground their definition of this.
