I'm assuming your not too far away from an interstate! Most fairly good sized cities will have a truck and trailer washing place.. they do aluminum trailers every day. The one down here north of Waco does an excellent job, and affordable! I just have to take the "chunks" out of the trailer (I generally take all the crap and shavings out) and they'll do the rest. No, you don't have to take out the mats unless you want them to clean under them. I take them out about once a year.
They charge me $85 for my 20' aluminum. WELL worth the $$. It looks like a new one when I get it back. But plan to leave it there for a day!