Attention: Camper Challenged,
I think the closest place to Ft. Desoto campground (that would put up with us scallawags) would be the Magnuson Hotel Marina Cove. It's at the north end of the Skyway on the east side of US -19 (
www.marinacoveresort.com) It ain't quite "fabulous' but it's better than "OK". A good boat ramp (Maximo Park) is close by. Plenty of parking for boat trailers and tow rigs.
There are not many Mom and Pop places left on St. Pete Beach with dockage anymore. Tierra Verde and Isla del Sol are pretty much all townhouses and condos. You better be ready to "jump up off your wallet" if you stay on the Beaches.