Go to www.metalwax.com. Best cleaner and polish on the market for diamond plate and stainless steel. Easy to use if you follow the directions. If you chose to use the wax, and I do recommend, use very little. Diamond plate will shine better than new.
________ Gang Bang Japanese
If you can find a "18-wheeler truck wash" ,they sell a muratic acid mix, you put in a sprayer. Or some of them will clean it for @ $20 while you sit in your truck (which is what I do). It keeps those stainless and aluminum trailers big trucks pull shiney, and will do the same on your dog box/trailer. Just be carefull, if you do it your self with the sprayer and a hose, make sure you brush it on even, with an extended handle brush, and hose it off right after. Also wear rubber gloves, and saftey glasses, and old clothes.
I've tried the metal waxes and polishes they sell and end up buffing my butt off. If I had a 1 or 2 hole box I might do the wax thing, but if you have a large area to get, the acid, in my book, is the only way to go. I do put a little metal wax on the front of my rig so that bugs and debris will hose off easily.