I consider a campground in the traditional sense - a developed area where people come to pitch a tent and spend an overnight or longer in the outdoors. I'd add that it is traditionally in a location that is fairly secluded and far away from modern conveniences and urban areas. Many campgrounds allow RVs.
An RV park, on the other hand, is a place where you can park your RV. Some RV parks are very basic and others are much more extravagant. The more extravagant ones would be considered resorts (think all inclusive). Most RV parks do not allow tents.
A campground is usually located in a place that is convenient for outdoor activities - swimming, hiking, etc. An RV park can be located most anywhere, including in the middle of large cities.