Chasing old 4" Duct under floor to bath?

[LRPorter]

I want to replace the 4" furnace ducting in my 24' Excella Airstream. At one place between the closet and bath the ducting goes underneath the floor for about 18". The old ducting is intact but leaks in many places. Should I run new ducting through this run or hookup to the old ? I have stuck my arm in this opening and fear if I remove the old duct I will not be able to run the new through this gap. Bath warmth is a luxry but at times oh so nice.

[ericjan2]

I've just done this on my 87 290 motorhome. I think you're saying that it disappears under the shower back against the wall (?) and looks almost impossible to thread through. It is tough because there is barely room to get a hand in and under, but not impossible. I taped the new flex tube to the end of the old at the vent and pulled it through from the furnace side carefully and easily as to not tear the tape which would have left the new flex 'stranded' under the fixtures! It was hardest behind the kitchen cabinets as it's really packed in there tightly! Keep the faith. Its a dirty and frustrating job but if you have holes you need to get it done!