Our last round trip from the west coast to the midwest and back took three months. Our overall three month avg was $38 per night.
A lot depends on your route and the type of parks you chose for your stay. We seemed to find ourselves away from the mainstream a lot, but there we always KOA's available, but they're rate is usually higher than most and skewed our numbers. They might be a little higher, but it's sure nice to know they are there when you need a place. :)
We now plan for $38 per night on trips and keep the change if it comes under. Way better than projecting too low a cost and running short.