I'd NOT recommend any "sticky/adhesive" velcro patches, either the small, squares, small round "dots" or any strips on the interior wall of the camper. The velcro sticks really well to the paper-like material and if you want to ever remove the velcro, the paper comes right along with it, thus you have a spot on the wall that does not have the pattern that is on the rest of the wall (bummer, been there, done that). If you have a different wall surface, then ignore this message completely.
You could use a "non-sticky" velcro strip that you attach to one of the screws that hold the top of the wall in place (they are under the bed pull out, remove screw, then put velcro in place and re-drive the screw back in its original hole), let the velcro hang down long enough to attach to the back of the seat-back cushion which has either the "sticky/adhesive" or sewed on velcro.
The seat-back will not be tight against the wall, but you shouldn't have to touch with your hand to push it into place when you want to sit down. Your rear-end will put it into the right place.