Here's some pix of my 20 footer under construction during the winter of 06-07.
I used 1/2" MDO ply over the vinyl interior panels and secured them with #10 self drilling screws into the steel vertical studs. You can see that I placed an additional screw between the factory screw locations.

The 1/2" ply panel covers two adjacent trailer wall studs so I could bolt down each side of the E-Track.
The E-Track is mounted to the 1/2" MDO ply with 1/4" bolts and 1/4-20 blind nuts counterbored into the back surface of the panel.

I covered the MDO plywood panel with white self adhesive vinyl shelf lining. The overlay on the MDO offers a very smooth surface to apply shelf paper to.

The shelf support structure sit inside the E-Track pockets and are held in place by screws.
By using vertical E-track, I can move the shelves up or down at will fairly easily, and without any help.

The two 8 foot by 8 foot shelves are made of 2" blue styrofoam laminated with 1/8" hardboard top and bottom to make a very strong diaphram that will support anything you want to put on it. Theu are covered with 2" thick foam rubber and furniture pads for the time being. I intend to replace the furniture pads with indoor/outdoor carpeting in the future.

I have already transported two KingCats, a Super Bandit, an Ultra bandit, and a BobCat XL so far, with adequate room.
The Super Bandit was fully assembled.

Everything in this job came from Home Depot, with the exception of the E-Track.