The VW part is the cab and front drivetrain of the T4 Transporter. V-6 Engine, transmission were shared with Passat and some Audi models, one of the engine options.
VW sold the T4 (as Eurovan only) briefly in the U.S. Slightly bigger than a minivan, smaller than a full-size van, it struggled to find a market. For most of the world, T4 sales were carried by the commercial van, Transporter.
Tariffs made the Transporter uncompetitive to import and sales didn't justify local manufacture (which is how we get most of our "import" brand trucks).
Most VW dealers don't like to see the RVs come in the door, as they were set up to service subcompact to mid-size passenger vehicles, not equipped to handle a bulky 8000 pound vehicle. Unlike Daimler, Ford, and GM, VW does not have a subsidiary dealer network in the U.S. for larger vehicles to which they can refer you.
The Rialta still has a following of loyal owners, as do the more conventional class C's (Vista, Sunstar) also built by Winnebago on the back of a T4 cab.
Since Rialta is a C, you might find some owners on that forum. I was able to find owners when I was looking at the Vista, and we have one currently in the storage lot I use, taken out regularly by the owner.
If you are looking for a B on VW platform, Winnebago sold the Westfalia Camper on T4 Eurovan as a Winnebago, during the interval T4 was imported. Prices on those are holding up pretty well, and some VW dealers will take the van into the shop.
Before that, the Vanagon Camper from Westfalia, and first two generations of Transporters with conversions by Westfalia, Sportsmobile, and others. Sportsmobile was a large supplier of conversions on the second generation van. These are getting into "collectible" price ranges today when fully restored.