Any ducting you add should be 4 inch duct which you can buy at Home Depot or Lowes, its just Mylar dryer vent hose.
That being said the minimum duct requirements are as follows:
Heat Range Configuration
Min Total CFM
Min #Supply Runs
Its not uncommon to have to add at least one duct and if that the case we suggest you just shove it open ended back under the living area towards the back of the RV. Its above the insulation so there is no lost heat, just warmer floors.
if you have any technical questions you can always contact me at PH# 425-408-3140 or email@example.com