I've seen something on here somewhere about attaching the skirting to the bottom 2x4's, and at the top having it in a J channel or similar so that it "floats". Kind of the opposite of what you're suggesting. Your idea seems good as well, though. I had some metal roofing I was using as skirting around a house extension on my old place (not a MH, just an old addition without a foundation), and did something similar. I found after a couple of years the metal tended not to be in the same place and came apart at a couple of seams a bit. I'm in Canada, so similar climate. It might have been from ice, frost or house movement, but the metal didn't "float" in the gravel like I thought it would have. Could have been from the type of gravel though, and I didn't do the drainage pipe like you suggest; if you can, use gravel with really rounded rocks (like from a beach or something).