Our 344QB was prewired also. I put a 13.5 non ducted dometic unit and it's the non low profile unit. That makes the camper pretty tall but the dealer didn't have the low profile in stock. We were at the lake and the camper wasn't dropping below 80* (no shade), so I took what they had and installed it while at the camp site. Also check past the air vent ttowards the front of the camper in the bedroom.I bet it isn't blocked off and a lot of air is being forced down the nose cap.
2011 F350 Dually 13,300 GVWR 3.73 ratio
2013 WildCat 344QB with outside kitchen