I have had great success in improving the miserable AM radio reception in my Silverback 33RL by clipping on a retractable shortwave antenna wire that I had laying around.
I had tried adding a ground plane to no good effect, and then played around with a small wire attached to the antenna stub (that's all it really is on mine) with an alligator clip. That seemed to help slightly, which gave me the cue to try the shortwave gizmo, which retracts into a small reel when not in use. I simply clamp it onto the antenna and let the wire extend down to the ground, then unclamp and reel it in when I break camp.
Might not work for everyone, but it sure did for me.