Burl- How about the higher GVW requires lower gear ratios ( bigger gears ?) which fit under the rear of truck chassis but would be to big to fit in the space a transaxle has in a FWD. Just a guess. The differentials in heavy box trucks and semis are huge. In my experience overseas, the trucks had RWD and only cars were FWD which spread to the US in the invasion of small cars in the 70's. Maybe it's simple physics.
-My cousin in NC just picked up one of those old GMC FWD RVs this summer from Ebay. Has the running gear of a Olds Toranada with a 455 CID and TurboHydramatic 400 tranny. I guess when GM produced thousands of FWD Toranadas back then the unit costs for a bare drivetrain would have compared favorably to the cutaway van chassis, but now a big FWD would have to be scratch built and the R&D costs would break the bank.