I bought a 5th wheel so I can triple tow my boat behind it. I use my tow vehicle to launch the boat, and retrieve. All concrete ramps that I have used have rumble strips for lack of another description. They are not flat smooth concrete. In the spring and fall the ramps can ice up, in the summer they can become covered in moss. Neither ice or moss provide traction, hence the rumble strips. Most times those rumble strips are angled to allow drainage. These angled rumbled strips will make an uncontrolled decent/retrieve extremely difficult. They will grab that front roller and push the trailer off coarse. Ever try to pull a trailer out of the water with a wheel that fell off the concrete ramp? It isn't fun, and can lead to expensive repairs.
I would be really nervous about backing in a boat with the RV. If those tires break traction your floating, camper and all. Hence the recommendation for a front mounted hitch, its safer. Now the $100,000 question. How many times do you have to break camp to launch/retrieve the boat? The campgrounds I stay at don't have overnight docks. I must launch/retrieve every time I go fishing/boating. I'm not too keen on breaking camp twice a day. How about you?
Why is it a $100,000 question. Several options, trailer to haul a vehicle and boat behind the RV, go the 5th wheel route and triple tow, scale back and go with a slide in camper and drop it in the campsite, or really scale back and tent camp. Most the guys I fish with spend the night in a motel/hotel/cabin. I prefer camping.
There are lakes with sand ramps, and big rubber tires on the trailer will work, however you can't always count on gravity to work. Most of those sand ramps have a very shallow incline. Which means your pushing with something. If the RV gets stuck in the sand, there aren't many that can pull you out. An electric winch would be handy to get you out.
Just some things to think about.