When the clamshell couch on my '07 34 is open the seat slides out from the frame. When someone sat on the extended seat, the whole couch tilted over with a crash. The lag screws holding the back of the couch down just pulled out of the floor. I wanted to use through bolts but the heads would damage the gasket and probably the carpeting the slide rides on. I used pieces of angle iron to extend the frame of the couch back to the wall of the slide where the seat belts are bolted through. The angle iron (actually old bed frame) is bolted to the couch base and then through the floor at the back wall. I had to trim one of the seat belt brackets for clearance. WRV used round head stove bolts as did I but that is probably not necessary. They do make it a one person job though. I used all four lag screws at the front (there are enough holes in the base) for a real solid mount.
Rick J Fisher, BW is Judy
2007 Alpine 34FDDS Limited SE
2003 Jeep Rubicon or a 1995 Toyota Tacoma