You may want to look at travel trailer or fifth wheel toy haulers. They seem perfect for using crates. The Logans have a good idea. As others are saying, you won't be getting into most private rv parks but there are many other options.
I really admire what you do. I recently rescued a Sheltie in San Antonio who was pulled from a high kill shelter. He was going to be zapped the next day. The agency I used is very involved in rescuing dogs and then transporting them to other areas of the county. San Antonio is evidently one of the areas with a huge "surplus" of dogs that get killed. I never knew there were people like you that transported them to other areas so keep up the good work! :)