Whatever size refrigerator you're thinking about putting in there, get the next size up.
Our stove is indispensable. We use big double-boilers to keep our hot sides hot without burning them. Our griddle is just another flat surface on which to put stuff. It gets used occasionally if we cater a breakfast, but other than that it really doesn't get used.
At least a three-well steam table. It can be configured all kinds of ways with half, 1/3, 1/4 or other size pans to hold a variety of items.
I wish we had a warmer installed in our trailer, it would have made things easier.
Shelves or cabinets. You'll never have enough storage space. Cabinets are nice because then you don't need 3800 bungee cords holding everything in while you're moving.
100 lb. propane tank is nice, two would be better. We only have one and get ours filled every week.
Serving window. Get it on the back of the trailer if you can (some venues charge you by the amount of frontage space you take up. Eight feet is a lot less than 16 feet.) Get a quality window installed with a screen that slides side-to-side. Those cheap storm windows and screens that slide up and down will kill your fingers eventually.
I'll probably have more ideas later.
