What I learned in creating this thread:
* Our interstate system is more extensive than I really realized.
* Our interstates follow a pretty strict guideline regarding their numbers, but it results in a lot of "split interstates" that are never going to be connected.
* There are more campgrounds close to the freeway than I thought there would be.
* Some campgrounds appear close, but the type of access road will slow down the traffic.
* Some states have a lot of state parks and national forests; others have very few.
* Some states put more into their state parks and recreation area websites than others.
* Many, many state park campgrounds are available through ReserveAmerica.
* Some national forest campgrounds can be reserved through ReserveAmerica; others are first-come, first-served.
* Quite a few state and national campgrounds have a 25' RV limit.
* Some state parks charge extra for non-resident campers. Camper beware!
* Some state parks, especially those in my (now former) state (California) can cost as much as a nice spot in a commercial park.
* There are probably many city parks with an RV area that are not listed. These are more difficult to find; inquire locally.
I hope that the information in this thread will be helpful to those who travel Interstate Highways and wish to camp out along the way.