Hey Grumpy, we had the dealer install a stackable unit when we bought our new FLSA in Jan. Prior to our first trip, I questioned theinstallation and was assured it was solid. Unfortunately, as you suspected, the position all the way in the back created some extreme biuncing and my unit moved pretty severe. Fortunately it is such a tight fit, it could not go anywhere but it did twist the original mount and vibrated one of the lines loose which we learned as soon as we turned on the water pump. I have an AWESOME dealer and as with every other problem I have had, he fixed it immediately and correctly. If you are ever in West Texas, check out Jerry's Campers in Abilene.