Excellent modification moving your lithium battery to the bed box.
Surprising you found the furnace duct tubes crushed after removing the underbelly cover. Our 25RB is our fourth travel trailer. We found the furnace to be useless in all four trailers primarily due to the blower noise and high amount of propane consumed.
When dry camping in cold weather we use a portable Camco Wave 6 propane heater. For hookups we use a portable DeLonghi radiator. Both have adjustable settings, NO NOISE and nice even heat throughout the trailer. Better options if the furnace blower keeps you up at night.