What kind of heat and how warm do you want it? The cheapest way for three years is going to be just waste a little heat most likely. You don't want a vapor barrier on the inside with a vapor barrier on the outside (aluminum).
Reading this a second time, the ceiling is bare metal? Ideally you'd add an arched roof like a mobile home and blow cellulose up there. If you want something quick to make it feel a little warmer and cut down on the dripping from condensation, add cardboard or celotex under the ceiling crossmembers, but only if the trailer doesn't leak. If it leaks, get a 50'x10' of plastic or used lumber tarp, or billboard tarp, and hang is sloping across the ceiling. Like one 1x2" the length of the trailer down the center to hold it up, and one on each wall a few inches down. That airspace will insulate well enough, and it won't drip on your stuff, just down the walls.