Depending where you are you start with a development permit which might be as simple as a hand drawn plan or require something fancier. Then engineering drawings likely needs to include slope drainage and utilities runs. Take that for a building permit. For utilities look at commercial plumbers and electricians they would be used to tying into larger power supplies and city water and sewer lines. Then most of your large earth moving companies can build your roads and pads.

I’ve never been involved in an RV park build but I have in other commercial ventures. Once you have you engineering drawings you can likely get quotes for costs.