Building an RV Park from scratch under 50k?

If I were in your position, I would research the zoning regulations for the county/township where the land is located. Also research what is required for sewer, water. What permits are required and the approximate cost. Print everything out - highlight the important parts and let them read it with their own eyes. If they still choose to go down the tubes trying the venture, there would be nothing more you could do. I'd leave it and them alone and let them do it their way with no further comments or questions regarding the project.