All of the above is correct in our experience. We have traveled in all 12 months of each year for over 50 years and are now using our 5th RV, a Montana 5th wheel.
Your average costs will be determined by how you pay, where you travel, and what your desires happen to be. Each year we stay at least 50 to 65 nights in Commercial RV Parks, and perhaps close to 7 or 8 free overnight parking areas, (truck stops, WalMart, business parking lots, etc). We travel for both business and pleasure.
Last year(2014) our total for the year, averaged out to just about $27.50 per night, just over $1800 for the year. We are life members of Good Sam (gets us the 10 percent discount at each GS Park), Passport America (gets us about 50 percent discount as each member park), and life members of KOA (same discount as GS), plus we have the Lifetime Golden Age Passport for National Parks, Monuments, BLM, Forest Service, etc.
On every trip we almost always stay at one of the parks where we enjoy the discount. Even at these parks, it also is determined where, (California, etc, are always higher than others) and the "Resorts" (we stay at a few on a regular basis) are almost always higher price than many other RV Parks. However, there are a few Resorts, and a few other parks, where we get an additional discount for the reason that we are regular, long-time customers of those parks over the years.
I know that over the years our Lifetime Memberships as mentioned above has saved us Thousands of Dollars in Park discounts, in addition to discount fuel and other costs. You may wish to check into this if you intend to travel enough with your RV.