I, too, had to make adjustments to the ducting to get sufficient airflow, in my case *at altitude!* Furnace would not start at altitudes around 9000 and above. Anything that took off back pressure on the single fan of the dometic furnace got it working. I wound up removing the louvers and replacing with a simple metal mesh because I didn't love the idea of removing the metal ducting entirely (even though the torturous path it goes through is clearly a design problem (there's absolutely zero reason the duct isn't located near to the furnace output...FWC could simply design/buy a duct with the opening closer!)
I've met other FWC owners on the road who appeared to share the same issue.
The other trick we used is a low temp thermostat so that, at night, we can leave the furnace on, but keep the camper at ~45° instead of ~60°F. This reduces cycling, but not as much as I might have hoped. Together, I can confirm we get about three nights of battery life with two AGM batteries, and little other use (phones, iPads, LEDs, that's it.)
Good luck, stay warm out there!