We buy and sell semi trailers and trucks. Time is money, so.. the eraser is the fastest and best thing I've found... It also won't damage aluminum or automotive paint. There is a best method to it, but I don't think I can explain it. If you have an air operated die grinder, get the extension/adapter. You can burn them off in a hurry with that.

Old, sun faded decals are almost impossible to remove from bare aluminum. The eraser wheel is by far the best method I've found. If the adhesive is so dry that the eraser won't remove it (which is very rare) we soak the area with PB Blaster, allow it to soak overnight, then eraser it off the next day. PB Blaster is better the best thing I've found to soften the glue. Not WD40, or other spray oils.

Here's my best effort at explaining the method. If you're holding the drill in your right hand, with the eraser facing left, the drill should be in the forward gear and the highest speed. Start at the top, working across in a line, that will push that row down and leave a clean strip. Drop down and make another stripe right at the bottom of that one. Work your way down. If you're left handed use the same process but run the drill in reverse. Clean at the top, dirty below. It will leave some debris on the aluminum that will usually wipe right off.