I'd like to spend a week or two each month in a National Park as this will help keep expenses low.
Don't overlook State Parks or City/County Parks, there are some good ones and many are less expensive than commercial campgrounds. The maps at RV Park Reviews are a pretty easy way to see what options are available along a particular route or near a specific destination.
When you hit age 62, be sure to get the National Parks Senior Pass ($10 lifetime) which provides half-price camping at National Parks and other Federal campgrounds (Corps of Engineers, Forest Service, etc). Almost worth postponing the trip just for that!
