I went out and purchased three different snips that I was going to use on my aluminum. When I actually started cutting I realized I don't have the physical strength required to cut through the stuff by hand. My snips were of no use to me at all.
I used a jigsaw. But do like everybody suggests and mask off all the material near where you are cutting. If you don't there will be nice parallel ruts in the surface of your siding and you will be really sad.
By the way, did you get the word to NOT glue down your aluminum? Especially in the Sacramento area. It will get real hot in the summer and the aluminum will expand at a rate different than the wood sides of your trailer and it will want to bulge. Gluing won't stop this "oil canning" it will just make it impossible for the aluminum to return to its original shape when it cools off later in the day. So remember, NO GLUE under the aluminum skin.
Could somebody else post the name of the flexible caulking that should be used under all the aluminum trim and screw heads to make the installation watertight?