We own a lot at Venture Out Resort in Cudjoe Key which is at milemarker 23. It's not strictly an rv park. There are small older trailers, mid rise, high rise houses and scattered RV lots.
They are all privately owned, so you would need to book directly with a Realtor. The park is very nice, gated, large ocean front pool, Atlantic side, marina and boat launch, tennis, hot tub....any amenity you could imagine.
There are a few Realtors in the area that handle Venture Out rentals for the owners. Dolbery Realty is who we use. There are others.
Another park is Sunshine Key between Marathon and Big pine. We stayed there every year until we bought at Venture Out. It's nice in the winter, not so nice in the summer.