J.S.
My first response is that your urgency to leave sounds a bit like a mid-life crisis mixed with winter doldrums. You've got three pretty substantial reasons why this is not a great idea at this phase in your life. You seem to be considering risking a lot, for what? So that you can "be living the way we want, doing what we want, working where we want"? I'm sorry, but that's what you do when you're in your 20's and childless or your children are grown and you're retired. Do some people die, get ill or become disabled in their late 40's and early 50's? Sure. But that could happen any time. There is really nothing in your dream that can't be put off for 4-5 years, until you son is done with high school, you can get your mother settled into a care arrangement that works for her, and you can save up to be able to purchase insurance for you and your husband. To go into your pre-retirement years without heath insurance is crazy.
Use your dream as a way to start a really, really great five-year-plan so that when the time is right, you're able to look ahead and enjoy every minute of your new life, not stressing out about whether or not your child is feeling isolated and resentful, about whether or not your mother is being cared for, about what would happen if heaven forbid you or your husband became sick or injured and you lost everything and then had to come back and be a financial burden on your grown children. Please also consider the quality of life that your 13-year-old would have. I have two 13-year-olds and living in an RV would be their biggest nightmare. But even if he has your adventurous spirit, please consider the nature of relationships that he would be able to have living at a KOA. Not to be snobby but we have a KOA near us and the "residents" who are not retirees are kind of sketchy, transient, families-on-the-move-living-under-the-radar types. I would think it would be difficult for him to find quality kids to be friends with and to be able to maintain those friendships for more than a short period of time, and for teens, a good peer groups is so, so important.