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.
Same problem, weak fan. inspected the duct work and I could not see anything out of the ordinary. I was wondering if this is the way it is or is there a problem/solution. I have no way of knowing if all of them are this way.
Yes this is an old thread, hope someone can help.