I had a similar issue and found that the ductwork and become separated from the furnace and was blowing most of the air into the covered belly. I had to remove the furnace and pull the ductwork back up and reattached it. Then just for a little overkill I fabricated a sleeve to insure an airtight connection. I also reduced the size of the opening that supplies the warm air to the underbelly.
Goody Two Shoes and the Filthy Beast
2008 Chevy Silverado 2500HD
Duramax and Allison 6 speed
2012 Wildcat 282RKX
You can't make things idiot proof, you can only make them idiot resistant.