RV Life, Good Sam and The Dyrt Offer Apps and Websites to Make Finding the Right Campsite Easier
I also use Recreation.gov
I really like Sēkr.
I've used the RVParky web site (rvparky.com) and associated app with decent results. National, state, and even regional park sites (via Google maps) have worked out as well. Unfortunately, like with EV charging, there's no "one app to rule them all" yet.