I've been wanting to tackle that upgrade. However, I have had too many other issues to deal with. I just replaced the landing gear which was too weak and slow. I just repaired the bedroom slide where the bottom of the slide seperated from the walls of the slide which allowed water to run along the metal trim at the bottom of the slide into the presswood floor and into the bedroom itself. Poor, poor, poor workmanship and design! Now I have a slight leak around the cabinet near the living room slide. I think it is coming in from the power cord receptacle but haven't pinpointed the culprit yet.
Anyway, I'd like information on the heater duct upgrade too! If I tackle it anytime soon, I'll let you know.