Pictures of some of my favorite mods

Finally finished my 12x70" wire shelf addition over the back storage cabinets in our T@B 320S, a la @Dalehelman.. 

One thing I did differently was to anchor the front corners into the sturdy wood shelf brackets over the side windows..

And by a happy accident, when trying to anchor the back corners into the side wall, I encountered the aluminum frame, such that all four corners are now mounted into very secure structures..
(The plastic anchor below was cut with a Dremel tool to support the shelf from underneath.)

This accomplished my other major goal to have the shelf high enough to avoid banging heads when sitting down in the back seat.  As a result, and to avoid putting screws into a thin ceiling panel, I made these custom brackets from 4-inch strips of common shelf brackets, and bolted the snap-on plastic shelf supports to the top of the bracket.  This allowed the bracket to be screwed into the solid wood cabinet frame..

For several reasons, there is a slight backwards tilt to the shelf.  First, I consider this an advantage, as items on the shelf would be offered a bit of resistance to sliding forward during braking, though it's really not noticeable when standing inside the trailer.  I also wanted to have the front edge higher to avoid banging heads.

More importantly, on the passenger side below, you can see the bottom of the wooden shelf bracket doesn't  extend as low, so this was the lowest I could mount it for max space.  I really didn't need to put the two side wall supports there, but they were done before I discovered the rear corners would be mounted into the aluminum frame.

The center brackets I made may not be the most attractive, and took some extra time, but they seem to work well, and provide the needed strength and more headroom.  I'm confident this shelf will be strong & give us a lot of extra needed storage.
The front edge of the 12" deep shelf is only about 6" below the ceiling, and we plan to attach a 4" tall stretchy cargo net over the opening to keep clothing items or small boxes of supplies from falling out.