The 1999 Tiffin Allegro we just traded in had an overhead pull-down bed, and it was attached in a pretty straightforward manner. The metal box/housings on each side of the bed, that contained the spring-loaded mechanisms that supported the bed, were mounted to the interior side-walls above the driver's and passenger's heads. Each housing was held in place by about 10 screws that were screwed right into the wall. I think installing a similar type bed would be equally straightforward, but you'd need a couple of folks to hold it in place while a third person put the screws in.
Something else to think about is the loss of overhead storage space and the innability to view an overhead TV. The bed and mounting mechanisms take up so much space that no overhead cabinets can be mounted over the driver's and passenger's heads. The overhead storage cabinets over the front windshield in our Allegro were for looks only, There wasn't enough room to open them when the bed was stowed so they were just screwed shut.
Kev