You could try:
They are located all over the country and, give a 10% discount once you become a member.
If you are a 'Coast to Coast' member, you can find campgrounds all over the country for abt 6$ per night stay. Drawback, is the initial cost of ownership in your "Home port".
Coast to Coast
You might try picking a WalMart Rand Map, usually has listing for campgrounds along the way or, stay at the Wal Mart parking lot for an overnight stay. Downside; No hook ups
Look for Truck stops such as:
They have about 150 plus location all over the country and, love to have you spend the night there. downside: Can be noisey at times..Plus: The best prices on fuel~!!
Another one would be Cracker Barrel. It's well known that they do encourage RV'er to spend the night at their location and, the food is always 'Great'.
Last, try stoping at rest site along the interstate and, picking up the instate campingground list that are for free.
Campgrounds are usually located right off the interstate~!
If you have a RV Center near you, try looking for a Woodall's campground listing book. List tons of locations along the way as well..
These are but a few in a long list of sources that you might find.
Have a safe trip~!~