Commercial RV parks, especially membership parks where many members go no place else, have storage parking areas. Seasonal RV parks often let youkeep theRV on your rental site. Most storage like this is out in the open, with respect to the property, so security depends on how well attended is the property,
I've not encountered much storage at actual campgrounds, as opposed to RV parks.But i'm in a campground now where the controlling authority (a tribe) allows some people to store on site during the week and come out on occasional weekends to use the site. This makes the CG look more like a low price trailer park, and wasn't policy when the state, and the COE before it, ran the campground, both had 14 day limits. But it seems to be what is needed to keep the CG well enough occupied to pay for upkeep.
If you want covered storage (almost a must here in thunderstorm alley) you'll most likely find in at a storage business.