Looks to me as he is turning a fiberglass trailer into a sticky! LOL
Pretty sure the trailer he is redoing did not come from the factory with that amount of wood framing or that type of insulation.
Looks to me that it may be a Beachcomber
he is rebuilding - which are fairly common in the part of the country he is located. As others have suggested some brands of fiberglass trailers do use "some" wood on the inside - around windows
and some wall framing etc - not Boler or Scamps though. But what is in the photos is way more wood than I have ever seen inside any fiberglass trailer.
If not mistaken the older Bigfoot
17' (as well as the larger Bigfoots) actually use a foam board type insulation (stuck to the fiberglass?) and not fiberglass batting as in the photos and certainly not all framed in with wood as in his photos.