Seems like a waste to get a van and then put an RTT on it. The whole point of a van, IMO, is that it gives you all that space inside that is sheltered from the elements, wind-proof, and "stealthy" as well (makes stealth camping in parking lots easy.)
I didn't realize until a few years ago that Previas were RWD. Not sure what kind of AWD drive train they have but I don't think it can be the same as the All-Trac on the Corollas and Tercels, can it? Both of those vehicles were FWD with a transaxle sitting over the front wheels while the Previa is a more "conventional" layout with a front engine and rear drive.
In any case, I'll echo the above WRT the kind of condition you generally see Previa's in these days. They seem to have become the modern equivalent of the 60's Hippie Van. Still, a nice Soccer-mom driven example would make a decent light ExPo vehicle IMO.