I have added camping/RVing up for our family, but per year as opposed to per mile.
We both work and have school aged kids, so for us, it's a hobby; we still own a home, etc. We use the RV as a portable hotel to go to new exciting places, and we use it to go 15 minutes down the road for a weekend to sit by the fire with friends.
For us, we get 1-2 BIG week long trips out of state (ie-we just returned from 11 days in the Smokies), 1-2 week long trips in state on a great lake, and probably 6 weekend/extended weekend trips during the year.
All of the campsite rentals, gas, excursions (horseback riding, amusement parks, etc), camper payments and insurance for a year costs us about 4-5K. If we kept our old paid off camper, stayed only at state parks and didn't do excursions, it would be less.
Most people I know spend 1 week at Disney for the same price. I think we're making out in the deal.