Yes, there should be ducts in the ceiling all ready. The spot in the ceiling is normally marked by a plastic rosette screwed into the center of the spot for the AC. Most people that put in the extra unit do NOT use the ductwork. You will get a faster cool down with the "dump" method of supply of cold air. When you hit a campground at night and want to cool the rig, the dump unit will put out colder air faster. You would normally run the LR unit for night usage.
2003 Dodge HO 6 speed Laramie Dually, Jordan controller, RDS fuel tank, Air Ride 25K hitch
Carriage 36' fiver, Onan 6500, Prosine 2K inverter, 3 slides, MORyde IS and Kingpin