Our experience having been RV'ing for 51 years as of this year is less than $30 per night FHU today. We've used Passport America 50% discount for years wherever it makes sense to and it's a nice CG or the Good Sam etc 10% discount. Also had a Coast to Coast home park and membership that really saved but doesn't save much anymore as the system has changed and we'd dropped them 3 years ago because we own the deeded sites anyway. We are not fulltimers as we have a large home on the lakeshore in Michigan and will never sell it as it's as fine as it gets, so why would we?
Over the last approx 5 years, our average CG stay cost has been an average of less than $25/day for CG's, both FHU and/or electric and usually water. Being over 65 and having senior discounts makes many state parks around the country and national parks real cheap per day. Really something to consider if you're retired!
For instance: The gorgeous Platte River CG in the Sleeping Bear Dunes National Lakeshore in Michigan is $13.50 a night total with 30 or 50 amp and a huge paved pad campsite. Very popular Tahquamenon Falls State Park 3 CG's in the U.P. are $22/night with 30 amp and $24 with 50 amp. Very clean modern new bathrooms at all 4 of these CG's and most of the state's state park CG's too.
Where and what time of year will make a difference also on the average site price so it's hard to just pin down an accurate cost/night. East Coast state's and West Coast state's CG's are more expensive than most of the rest of the country for equal type CG's. By the week price cuts cost also. Our very nice FHU with 38 cable TV channels hookup and excellent WIFI campground resort we go to in Florida in the winter for a couple months is a whopping $10.17 per day by the month with Florida sales tax included. Now that's cheap!