It's been a while but we finally towed the trailer home and undertook the rodent damage repair. I was able to pull one duct out and replace it without getting underneath. That was the easy one that didn't have any of the insulation in it, just organic mouse deposits. The one that I was most concerned with I could not pull free. It was running through a bulkhead in the frame about a foot forward of where it went through the floor from the furnace and seemed to be hung up there. I could get it through that bulkhead but it still wouldn't pull forward from the floor vent. This was the bad one that Mickey had chewed through and built his cozy home. So, taking Chuckster's advice, I went under and planned to drop the coroplast from one side and find out what was holding this duct in. What I found was the propane black pipe running the length of the area I needed to drop eliminating that possibility without dropping the plumbing. Plan B. Measuring carefully I cut a 1 square foot opening, 3 sides only, thanks to all of the knowledge I have gained on this forum, and pulled it down enough to see what was going on. What I found was nothing appeared to be holding the duct, but I still couldn't pull it through in either direction. At this point I could see the light from the vent hole in the floor and I knew I could get the new duct through to the furnace so I had nothing to loose. I had my daughter hold the end at the floor vent and I gave it a mighty yank from the underbelly hole. There was a bit of a tearing sound and it came loose. The two of us played tug of war back and forth a bit. It was definitely loose and appeared to be mostly intact. I taped a coupler to the old and new ducts and pulled it through with no problem. When we had the old out we could see where one section about 18 inches long had been against the edge of the coroplast where it is attached to the frame. Urethane foam had been used to seal the coroplast to the frame and had stuck to the duct, which was holding it tight. Needless to say, the new duct doesn't stick to that and is laying loose in the underbelly like the first one I replaced. The third duct running to the bedroom did not exhibit any of the characteristics of the other two relative to Mickey and his family living in it. What I did with that one was just cleaning it. I tied a piece of heavy string to a plastic grocery bag and pulled it through with a shop vac. Once we had the string through I tied a large loofah in the middle of the string and soaked it in disinfectant. We played tug of war again and ran that thing through several times. The first time it came out a little dirty but after 5 or 6 runs through it was clean. We buttoned everything back up and started up the furnace. Everything worked and smelled fine.
I also took a little time to wire in quick connects in case I ever have to pull the furnace again. A little extra length of wire and a few weather proof connectors and we are good to go.
Thank you Chuckster for your sage advice. As much of a pain as this was I would not have felt as confident in taking it on without you.