If you stay at state parks, you can go to reserveamerica.com and type in the state you are interested in. On the left-hand column you can then select that you are looking for a RV site, and enter the length of your rig. You can also click on "more options" and enter what type of electric hook-up you want, etc. Then hit search, and sites that match your criteria will come up on a campground map and list. If you stay at federal government parks (Corp of Engineers, US Forest Service, BLM, etc.) you can do a similar search at recreation.gov I have found that there are fewer sites for big rigs at state and/or federal parks, that's why we chose a Bighorn that was under 36 feet. We like to stay at the public parks, rather than the private parks, as we find there typically is more privacy and woods at the public parks.
If you are a member of AAA, you can also go to your AAA center and ask for the Woodall's official campground books. AAA gives them out free of charge. There are Woodall books for each section of the country. There are state maps and the campgrounds are listed by the cities they are closest to. So once you determine where you want to go, you look up the city/town and the campsites will be listed, both public and private parks.
Enjoy your new rig & your new camping experiences.
George & Linda
