If it is in or near a major city, the land necessary will be insanely expensive. Zoning will be challenge. Impact fees can run into the six figures. Public opposition will be fierce. Nobody wants a trailer park (and that is how the public views RV parks) in their back yard.
Construction costs are high to begin with, and building commercial building to current ADA, Safety and environmental codes make those construction costs much higher than the residential costs people are more familiar with.
RV park construction usually cannot be financed. Banks view such construction as highly speculative and unlike many commercial construction projects if the project fails there is no fallback usage for the facility. When the Southern Prussian cuisine restaurant fails, there is somebody who wants to rent the building for their Fire Grilled Grasshopper pizza concept. RV parks are pretty much only good as RV parks or to be used as raw ground for development.
The movie "Field of Dreams" had it wrong. If you build it, they will not necessarily come. You will need to advertise and build the business over time. During this time, the investor will not be making money and likely will be running at a loss. Deep pockets are required to build parks.
While RV Parks are profitable, especially for an owner/operator, they are not slam dunk cash cows. They can struggle. If overbuilt, they can lose money. If they are under built, they can quickly become unpopular. Basically, there is high costs, high risks and relatively modest rewards. These are but a few of the reasons that more parks are not being built.