Start up with the https://www.sba.gov/ they offer free advise on how to start up any business. Take the time to develop a business plan, It can save you 100'S of thousands if not millions getting started.
Like others have said visit some parks and get an idea of what you want to offer. Add that to your plan.
I have looked thinking I wanted to do this and spot checked in the remote sites of Maine and Alaska. You can buy some for less than a million. But I have to tell you you are dealing with service to the public, I can't think of a harder group to deal with. In our case retail was difficult.
Good luck and keep us posted.