If I built/bought a RV park in central Texas or on I-10, I'd probably be able to turn a profit. However, I know my limitations.
In a lot of areas, established parks got the land they were on when it was relatively cheap. They don't have to squeeze in customers, sardine style, in order to show a profit since the land was paid for long ago. Due to being grandfathered in, they usually don't have to make major expenditures to get up to code other an an occasional inspection. One big issue they face is Wi-Fi, and the cost of getting that up and running (which can be extreme if there is no useful upstream ISP.)
I've thought about combining things to help finance the RV park side. A motel comes to mind, provided there is separation between the two for security and noise reasons. Same with a storage facility (again separated from the RV park side by a high security, opaque fence for privacy reasons.)