I own a Tropical which is made by National RV and they are out of business. The fact they are out of business does not concern me one bit. The only real affect it would have had on me is if my factory warranty were still in place at the time they went under, but it was not.
The only thing motorhome companies really build is the house frame and maybe the end caps. If something goes wrong with the house frame you really don't need the original builder to fix it. The end caps could be an issue, but if my end caps get damaged then that's something that would be covered by insurance so that's a problem for my insurance company to figure out. I understand that National sold the molds to somebody so there is somebody who can make new end caps.
But other than what's listed above, everything else on my coach is made by other companies. Freightliner made the chassis, caterpillar made the engine, the tranny is an Allison, all the appliances are made by others, the furniture, etc...
So I personally would not be concerned over buying used from a company that's out of business, with the exception of if it's still new enough to have had the factory warranty.