Our very first RV trip was to Fort Pickens. We did stay in the A loop. It rained the whole time we were there, and I had the best time I've ever had camping in the rain. Even my BF, who loved tent camping, was thankful to have an RV to stay in.
The A loop is the one closest to the beach. It's also much smaller and easier to get around than the other sections. I still wouldn't call it "close" to the beach, though. If you're like me and have chairs, umbrella, cooler...it's quite a walk to the beach. They had just remodeled the shower house when we were there. As others have said, there are no sewer hookups. Make sure to visit the fort while you're there. They offer both guided and self-guided tours. If you're into fishing, their little pier is fun but usually crowded.
We're headed back for Labor Day weekend. We plan on touring the Naval base and the lighthouse while we're there this tie. Maybe it won't rain on us again. Hope you enjoy Fort Pickens.