I own a 24 foot RV built on a Ford E-350 chassis. Gets 10-11 mpg. Yes, as a previous poster mentioned it gets expensive. However, a 700 mile drive with a bathroom, fridge, and room for Mom, infant and dog to walk around, sure beats stuffing them all into a car or SUV.
Find State parks along the way. Usually, cheaper than commercial parks, and typically more "outdoorsy."
Again the cost is high, but the memories I have with the kids are worth it.