When we rented the Class A last December we stayed at the Majestic Oaks RV Park in Biloxi. It was incredible. Okay, it was our first RV park but so much prettier than the one we stayed at on the way home. LOL
If I remember, they had a few back in spaces that would fit a 45'. It's a very manicured park, all concrete. Each spot comes with its own palm tree or an oak tree. We were impressed with the dog walk areas. Nice bathrooms (you know... should your toilet start leaking halfway through the week and you need an alternate bathroom...not that that would happen, right? :\ ), showers, and a pool.
We drove through most of the other RV parks while we were down there because we were trying to get a feel for what RV parks are like. That one was the nicest. There is one (I think it's called Cajun something) between Biloxi and Gulfport that has a few ocean views (and plenty of room for a 45') but it was not as manicured or paved (if that matters to you.)
If you find you can't fit into either of those, there's a state park with RV hookups down south of Waveland that could probably hold any size RV. Ocean views. A little remote, but very peaceful. I forget the name of it, but it's on the map.
If you can swing it, drive over to Ocean Springs (not in your RV though... the trees are very low in this gorgeous little town) and get Tato-Nuts first thing in the morning. Best doughnuts ever. As the name suggests, they use potato flour.