Yup, I'm here!
I'm told Airstream purchased the design from a Canadian company by the name of Okanagan or something like that. In fact, there used to be a member of the old Yahoo group (prior to the b190.com site) that had an '86 camper that was identical to the first year Airstream B190 inside but wasn't an Airstream.
As you've noticed there were plenty of other brands making similar campers to the B190s in the 90s. Most of those brands are gone now; the only two I know of that survive are Airstream (who of course survived by building OTHER campers) and Roadtrek.
The 460 seems to be a pretty reliable engine. I've heard good things about the V10 as well. Some have the 351, but those are fairly rare.
As bryanl was saying, at this point, you have to keep in mind that you're buying a 10+ year old vehicle and a 10+ year old camper, so the usual issues with each apply. Even if it wasn't used much, rubber breaks down over time, fuel sitting in the generator's carb will gum up, etc.