smaller production numbers make them harder to find...
national craiglist searches might help..
sportsmobileforum.com
here on expo
samba.com for VW vans
often they aren't listed as RV's..
If you aren't concerned with offroad travel then I have often wondered about a diesel short schoolbus...you could take out the seats and block out some of the windows and be very creative.
Honestly I think the most important thing is to keep the vehicle investment (both money and time) low and simple....then just go!
People have traveled around the world in VW Bugs, Datsun coupes and on moto's....don't wait too long for the "perfect" solution...
You don't mention the age of your children or you & your spouses size...but a used short school bus can be had for cheap. Add a RV style self contained toliet, some folding camp tables & a folding camp kitchen, 4 camping cots with foam & sleeping bags, remove all the seats and bolt in 3 van style captain chairs that swivel and you are ready to go for dirt cheap.
Of course you can upgrade all of this before you leave or for a real adventure wait until you get to a cheap labor country and have a local there build out your interior after you have lived with it for a while and know what you want.