I painted my popup (needed something to camp in while reengineering the argosy) last year. It's a 82 starcraft with aluminum skins. I used Ace galvanized and aluminum primer. Then used Ace Rust Stop oil based enamel with a hardener. I sprayed it but considered rolling it. It came out great. So much so I'm going to put a custom paint job on the argosy for less than $500!
What hardener did you use? and what quantity/ratio?
I will be painting my argosy 24 with the (Canada) home hardware Rust Coat paint. it s made by rustoleum but the great thing is... it come in gloss base so you can costumize the color. I just finish rolling 2 test panels with a nice Subaru World rally blue and it s a very nice finish.
I use 50/50 paint/mineral spirit and it took 5 coat for full coverage with wet sanding between every 2 coat. It will be time consuming on a full trailer but you can get it done for 100$....