I've successfully used a low power electric pressure washer to get the big stuff off, then toilet bowl cleaner (Sno-Bol works great and is cheap).
Let the toilet bowl cleaner sit for a few minutes, scrub lightly, then rinse thoroughly. You may have to repeat a couple of times. When finished, put a couple coats of wax on.
Sno-Bol is very effective on rust stains, yellow fiberglass where stuff grew, etc. But, it's nasty stuff...don't breathe it, don't touch it. Sno-Bol apparently eats galvanizing away, so keep it off the trailer.