I had a similar issue with my 2014 FL36, there was no heat in the bedroom and bathroom. So I did the same thing, used a scope to check it out, but could not find anything. We then turned the furnace on, and there it was. The inside of the duct has a foil liner that let loose. With the furnace running the foil would act like a valve and stop the flow of air. When the furnace stopped, the foil fell down, due to gravity, and the duct opened up. We were EXTREMELY discouraged by this poor workmanship, but frankly not surprised. The only thing I could do is rip that piece off and flow was returned.
You should note, at least in my case, I did not need the scope. A small mirror with flashlight works just as well, if not better.
2016 Silverado High Country 3500 Dually Diesel 4x4