It could be a simple fix or it could be a major job, speaking from experience. Before returning to the dealer, you can make a quick check with a flashlight and a small mirror. Remove each register and using the mirror and a flashlight look at the ducting to see if there are any obvious disconnects. If you see a problem, you can decide if you want to try to fix it your self, or return to the dealer.
My experience a couple of years ago with a new Cyclone toy hauler was that there was very little heat coming from the registers, but the furnace was working. I pulled the return air flow register and found that the hole in the bottom of the furnace plenum did not match the hole cut in the floor to access the heater ducting, and the warm air from the heater was just recirculating in the cabinet where the heater was located. The solution was to remove the heater, cut a new hole in the floor to match the hole in the heater plenum and cover the original hole in the floor. I did this myself because I did not trust the dealer to fix the problem, based on previous experience.
