Finished this project. Like most motorhome projects it grew a little.
All of the ducts, not just the mouse chewed ones, had holes. Most were small but all would leak air/heat.
I used a combo of rigid elboes and newer mylar coverd flex. Tied string to the old ones nad pulled them out. Then used the string to thread the new in, or out as the case may be.
This was with a surburban furnace so after removing the furnace from the shell I could access the snap rings for the ducts and do the hook up.
Like most deals the second time would be easy. The benefit is no air/heat loss now and removal of 20 years dust.
2016 Bay Star Sport downsize from 2001 MountainAire
Toad GMC Terrain, might leave it home on some trips.