Missing heat vent 2018 coachmen..

I had to add 2 ducts before mine would run continuous and not cycle the burner on the limit switch. Adding 1 helped but not enough. I was timing the burner cycles on and off. Was a bit dumbfounded when timing after adding the 2nd and it just ran until the thermostat was satisfied. Tons of heat, I was sweltering.

I would move your new duct to the rear knockout and add the second vent to the left and connect to the forward spot.

I used a more ridged flex duct from Home Depot. (Dryer ducting) I think it flows better than the foil wrapped spring.