...the plan is to immediately replace the roof vents with fantastic fans. I can source the fans and install them MUCH cheaper than the dealer quoted so will do that.
PlaidCat, welcome!
Before you buy Fantastic fans and then fixed covers so that you can use them when it rains, check out Maxxair fans. Ours works rain or shine (does not close when it rains) with a built-in shroud. When lowered (for travelling) it's only a couple of inches high.
I purchased our NEO trailer from L&M Trailers in Woodstock, NB just before we left Nova Scotia to return to the west coast. Almost 2 years full-timing in it (Feb 15). No problems and no leaks with Maxxair fan and 2 - 14"x22" skylights in the roof.
Good luck with your build. Look forward to lots of pictures.