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Another suggestion for use on the receiving end: If the trailer has a large window with a view in your direction, and a non-metal shelf, table, etc. next to the window, the AM receiver could be placed on that. If the AM receiver has external antenna terminals, a small manufactured AM loop antenna could be placed on the shelf/table and connected to the receiver.
For more information, see “Receiving Part 15 Transmitters” in the Part15.US Library. DISCLAIMER: I wrote that. Proceed as you see fit.