If you are buying land for an RV to use. Ensure you are going to be able to get a permit for well and septic and that an RV is allowed by local code.
One way I have seen people get around this is if there is a house or mobile home that has burned. Sometimes you MAY be able to set up there, especially if there is an outbuilding ie, garage etc which contains the well and maybe a sink. Then you can often tie in an RV to the existing systems without running afoul of the permitting process.
As far as the permanent park, there are quite a few parks that are setup for 55+ folks to live full time. I was just reading about some in Mesa Az. yesterday after seeing some post on this site. Most seem to start about $60k and go up to $250k or more, for a lot and park home, but that does not cover whatever the HOA or management company charged monthly for services. There are a lot out there, but you will have to settle on an area and then start your research.