They are not exactly shifted on the Ford (or whomever) frame. The Ford frame rails are actually cut in two and a section welded in to lengthen the wheel base to meet the RV manufacturers requirements. The driveshaft in ours is one piece, so probably one made to specs, not a piece welded in. I'm not sure if brake lines and electrical were added to or replaced with longer ones. I haven't noticed any splices in ours, frame rails yes but not other components.
All of this must be done to meet Ford specifications.
Ours does have frame rail extensions behind the rear axle to support the body of the RV and it also does, or can change the towing specs. Ours is still rated to tow 5000lbs but can only have a tongue weight of 350lb.
Ken