I like the roll-up door. Good way to break out of your original idea for a solution to a nagging problem. It still need to be cleaned up a bit - put the metal slats on the door to make it a bit more realistic, that sort of thing. Agree with your boss about the plywood inside, much better than the metal look from before.
Also agree with rossable to look at a lot of the inside of trucks to get a better feel. Best would be to go out and take a hundred pictures of your trucks from every angle you can get, with as much light as you can get. Try to get them with stuff inside so you know exactly what you would see (even if you don't actually replicate that, you will know what it SHOULD be so you can take it from there). Here's one image that my search on "interior shipping truck" came up with:
I think you are doing an awesome job plugging away at this. A lot of people would have quit by now. Keep at it - eventually you'll get to the right point!