It could be as simple as the duct separating from the floor allowing heat to go into the underbelly.
I pulled a newbie and left my "Damp Rid" on the counter while driving. It hit the floor and dumped in through the register into the duct. I proceeded to remove the register to clean out the duct when I discovered the top of the duct had separated from the floor of the coach, allowing heat to escape. This explained why my rear heat vent was barely flowing.
See video for clarification.
2016 Cherokee 264L
2013 GMC Sierra Denali
EAZ Lift 1000 WDH w/Sway Control
2016 Nights Camped: 20 Trips: 7