Have you ever RVed before? If not, then you should rent one and see if you even like living in one. Go to a lot of RV shows and RV dealers and walk through many types and models of RVs. Look at the relative costs of each type. When you decide what type of RV you want (if you decide you want one) go to the forum for that RV and read a lot and ask questions. I would purchase used once you have the information you need to purchase. The cost per month will depend on where you park and how long you stay there and what your lifestyle is. For the costs for full timing, I would go and read the posts in the full time forum.
I think you have a lot of research to do before you buy anything.
