It sounds like one of those articulated double buses would be about right for a group like yours, but articulated buses are not likely as an RV. I'm sure you could turn around every time you got to an airport.
I've built for a family of 5 but that was 20 years ago, and I've moved on to bigger and better buses. That's great for a large family. It beats the standard passenger van hands down and the fuel mileage is quite similar to passenger vans, but with much more space and hauling capacity.
I'd suggest you take your best shot at purchasing a suitable bus and then try to camp in it right beside the house. Figure out what you need versus what you'd like to have in the bus. Obviously you'd be better off trying this in warmer weather, especially with the kids.