Just FYI, most of the low cost trailers I've looked at have the single sheet in the roof center.
And some just have partial panels on the side walls.
You can look at it two ways, it may seem cheap to you, but you get a much better look at the trailers "bones" which is never a bad thing!
And certainly makes it easier if you're adding lights, roof vents, etc.
$8900 is low cost? I get what you are saying about making additions easier. Only things I would probably add is a simple bench in the V and put my generator and portable compressor in it.