Sounds like the duct is blocked, disconnected, caved in or otherwise. If you can't see into the duct from the vent, try sticking a small digital camera down it and snapping some photos. (or video cam) You can see down all your ducts that way and possibly see any problems.
In the KEYSTONE OUTBACK line, they put water tanks by the floor ducts. When the water tank was filled, it squished the ducts and permanently restricted air flow.
Pete (Jolly Mon)
2014 Jayco Whitehawk 24 RBS
2004 Nissan Titan LE, 4x4