Heat Duct Assembly Mystery Hole Under Tub

[cadoc]

In the process of lighting the NT30 in my 31 ft Sovereign (after 12 years of non-operation) I pulled the heat duct covers for a little vac and found a 3" hole about 12" inside the rear heat duct. It is mis-aliegned but clearly has an aluminum hose going somewhere. The manual shows and says nothing about this little goodie.
I imagine it may help keep water lines inside the rear access door from freezing. If it is OEM then I will assign more importance to getting it lined up properly.
Has anyone ever seen one of these?
The furnace lite like a champ.
Doc

[cadoc]

I found the right thread, it's to the black water tank.