We've done it a dozen times but never tried to do it from an RV, I don;t know how you would even accomplish that other than having a person who drives the RV full time. A lot of the GAP, especially beyond Connellsville PA, are not accessible by road.
There is KOA a long days ride out of Pittsburgh (a good 50 miles) that the GAP runs right through. It's named Uniontown KOA but is actually located 3 miles from Connellsville PA (A town on the trail) and Uniontown is about 12 miles away from the campground so I don't know where the name comes from. It's also known as Rivers Edge however the CSX main line is 100 yds away across the river, and a train goes by so often you'll never even notice it. That CSX main line is across the river from you even if you bike camp along the trail, from Pittsburgh until you almost get to Big Savage tunnel, a good 120 miles.
Other than that it's slim pickins for RV capable campgrounds if by that you mean full hookups? There is a state park campground in Ohiopyle PA about 23 miles trail ride from the KOA mentioned heading towards Cumberland, but it's not right on the GAP, about 4 or 5 miles away and up on top one hell of a mountain above the river and trail, no full hook ups, electric only. There are some other private ones nearby but they're some miles drive from the GAP.
Moving east probably 12 miles in Confluence PA is a Army Corp campground about 1/2 mile off the trail called Outflow Rec area, a few electric sites, but most are dry.
Further on near Rockwood PA a good 20 trail miles are some private campgrounds with hookups and a couple of state parks with a very few full hookup sites, Laurel Hill SP and Kooser SP. All of them are a good 10-15 miles drive from the GAP trail however. Pioneer Park and Hickory Hollow are the private ones I know of. Steer clear of "Scottyland", it's a true garbage dump of a campground.
Past there the rest of the way to Cumberland I do not know of any other campgrounds that would accommodate an RV with or without hookups that are anywhere close to the GAP. Beyond Confluence to Frostburg MD you're up to it deep in the Alleghenies and off the beaten path for the most part.