I bought a Coleman 6 person Instant Cabin, and DS and the grandkids slept in it when they went camping with me last month. It worked well. No reason why you can't take a tent along with an RV.
The reason why I chose such a large tent was past experience: tents never hold as many as they claim, unless you cram in tightly. A 6 person tent holds 3 or 4 quite comfortably, plus floor space for gear. And if I ever feel like going tenting myself, there's plenty of space for my cot and my zero gravity lounger and whatever else I'm lugging along.
For just your son, pick whatever tent suits your fancy!