I've seen quite a few around here, most are not in great condition. Most common chassis seems to be the late 70's Toyota but it's not uncommon to see them with a (4th? generation?) 1984-88 Toyota chassis. Every one I've seen has been a 2wd and the vast majority have some variation of the 4cyl engine (20r or 22r, I would guess.)
Probably easier to find than a VW camper and I'd guess they'd be about even on performance (i.e. slower than molasses going uphill in January.) Definitely cheaper than a VW camper because AFAIK there's no "cult following" for Chinook campers.
I've seen photos of 4wd versions but never seen one "in the flesh." I don't believe the Chinook was ever offered from the factory as a 4x4 so my guess is that someone grafted the camper portion onto a 4x4 chassis. If that's the case I would be a little worried about stability and strength since the 2wd version probably had a beefed up frame to handle the extra weight.
Not sure what MPG would be. Again, my guess is that it would be disappointing. For one thing, I think most of them had 4 speed rather than 5 speed trannies (IIRC the 5 speed did not become common until the early 80's.) Second, that little 20r or 22r motor would have to work awfully hard to pull all that weight.
Still, they are funky looking little vehicles. With the price of gas one has to wonder how long it will be before the RV industry starts to adjust and produce more RVs based on small, efficient platforms.