You need an unvented roof.

I'm pretty sure a unvented roof is called a hot roof. Ceiling/walls finished up against the rafters.

If you are not going to use the space from the upright 2x4's to the slanted part of the roof and enclose that you don't insulate against the roof and then have vents under eaves to let air in and vents higher up for air to exit .

Here is a link to some hot roof info, there is a bunch of info on line.

https://structuretech1.com/hot-roofs-minnesota/

I wonder if the company that built the modular home gives recommendations on how to best finish that area?

My BIL finished off the walk up attic on his cabin way up in northern Minnesota with a hot roof. He would have rather used the spray foam but didn't want to spend that much. He opted for the fiberglass batting vapor and finished off with plywood panels rather than sheetrock. He didn't want to bother with taping drywall so he went with the "rustic" painted paneling look. But that's fine for a upstairs sleeping area anyway. The cabin itself was built about 8 years ago, it looked similar to how your rafters look before he finished it.