Here in Florida I see road-side ads for RV-related work including remodeling, maintenance and repair. Considering the generally poor quality of RV construction and the number of snowbirds down here, I suspect they stay busy, at least in the winter months.
I have three kids and that's pretty much what we've done, though we've yet to do a trip longer than 6 weeks. It's a pretty good compromise, but we have younger kids (2, 7 & 8) so YMMV.
You've gotten good advice so far. If/when you rent a comparably-sized vehicle, consider a worst-case scenario, like being stuck inside for several days because of weather. Will everyone come out of that small box alive and sane? :)
You haven't really mentioned how you intend to use the...
Another option is to buy a used 14 passenger shuttle bus based on a van cutaway chassis. Here's an example (PDF file). You'd have to convert the interior, but you'd have a shell with windows, plenty of height, and good interior width. Some have luggage space in the back, or a doors for a...
I don't think you can buy one anywhere, but if you really need high capacity there the MZKT-79221 with a payload capacity of greater than 100,000 pounds:
In the "price is no object" category you can get it's little brother here which has a similar payload.