I run a site for B190 owners (b190.com) and I've been watching this with some interest myself for years. I should note I sold my B190 just a few weeks ago, albeit for a bit less than I'd hoped to get for it.
First, Airstreams (trailers and motorhomes) always seem to be low in NADA. You can easily confirm that by checking prices on eBay and comparing to NADA.
To actually answer your question, though: you're right about the hoses and all getting old (though I never had to replace one on my 1991 B190 with 122,000 miles). Hopefully the tires have been kept current - they should be replaced every 5-6 years regardless of mileage.
For the prices, I sort of see where they are coming from - a new B190 in 1991 was something like $50K. (I don't know what they were in, say, 1999, so I'll use this example.) If it's in barely-used condition 10 years later, and a new Airstream B van is $100K, $25K starts to look like a fairly good deal.
That said, I don't know that I'd pay $25K for one like that, though. A lack of use can be hard on a camper, too - seals dry out, etc. In addition, if it has a generator, they're supposed to be run 4 hours a month minimum...I doubt anyone actually does THAT much, but if it sat parked most of the time, they probably didn't run the generator much at all, so who knows how well it'll run and last?
1995 Airstream Classic 30' Excella 1000
2014 Ram 2500 Crew Cab with Cummins 6.7L Diesel
Sold but not forgotten: 1991 Airstream B190
Sold: 2006 F-250 6.0L Powerstroke Supercab