I've had aluminum roofs on a bunch of race trailers, and the aluminum itself is maintenance free, that is true. The typical installation method on those has purpose built aluminum trim that screws in from the side and clamps the edge of the aluminum down so there are no screws through the roof, and a heavy bead of self leveling sealer all the way around the edge on the roof. That sealer will need scraped off and reapplied in about 10 years in my experience. Which is just a time consuming pita to scrape off, and I think I used about a dozen tubes of sealer on my 40' trailer the last time. Rubber roofs are more industry standard for RV's with the curved edge of the roof no need for that big bead of sealer. In the long run you'll spend way less time on maintenance vs. the annual clean and seal to do a rubber roof properly. I'm sitting in a camper with a rubber roof right now, and spent a lot of time in race car living quarters with an aluminum roof, and the biggest difference inside is the rubber is WAY quieter on a rainy day.
I used aluminum on the roof of my small toter when we built it. Comes in a big roll cut to your length from your friendly local semi trailer repair/dealer. I don't remember the price but it didn't sound bad at the time. Of course my truck has a small roof. One piece is the only thing to consider, don't even think about sheets and seams on the roof. They should be able to get you various widths, and the sheets for true semi trailers are oversized and they have a tool that bends the edge over so everything overlaps.
If you want to walk on it you will need something substantial under the aluminum, trailers with a "walk on roof" option come with a layer of 1/2" or 3/4" plywood depending on what you are doing up there. It's fine to step carefully from beam to beam to do occasional maintenance, but if you planning on climbing up there and putting up a bunch of na$car flags or some such every weekend it will be bent down between the beams and making puddles in no time.
