mildew on exterior walls; none on open-vented interior walls

- I meant to ask this question earlier, as I started cleaning and preparing my trailer, after sitting untended in the garage for 18+ months. I noticed a little mis-coloration of my paint, while working on the trailer, in my badly lit garage bay. I haven't put the big halogens in the bay, yet, for better lighting, 'cause I'm using them for my room rebuild inside the house. I grabbed a nearby kitchen wipe (I have them stashed everywhere), and wired a foot-square surface clean, until it shone in reflected light. The wipe was brown and lightly green.
- I live on a sandhill, and our 8 dogs and the neighbor's horses stir up a lot of dust, so I expected the brown. But, is the greenish tint indicative of mildew? Why on the walls and roof of the trailer, unless it formed just after I backed it in 18+ months ago? I had just hosed it down, and put it away wet. I hadn't even opened that bay for over a minute or two at a time, just to reset the compressor or the GFCI breakers, in all that time, until recently. The trailer's exterior vents were left opened, and my garage is the metal shed kind, not sealed either, so I expected dust and mildew inside, too. But, there was neither. I wiped floor, walls, carpeting, sniffed the uncovered mattress, and noticed just a slight mustiness, but no dust, dirt, or mildew. My garage has spiders, scorpions, occasionally mice, rats, and snakes, but the trailer hasn't been bothered, just the mildew on the outside. Lucky, I guess.
- I thought I'd never get back to camping, my health made me retire a bit earlier than I wished; but my health improved, so my trailer will get some attention, now. I won't neglect it after this.