It might be to heat that area if the water pump is in there. My Rockwood 5th wheel has a duct that dumps into the belly where the tanks are. Just one caveat, before plugging the duct to increase flow to other areas, look at the manual for the furnace and make sure that there is enough ducts coming off of the furnace. Mine manual saves something like 3 or 4 4" runs as a minimum.
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.