Travel Trailer blocking RF (Garage Door Opener)

Like you say, a big hunk of aluminum by itself should not be causing any problem. Try disconnecting the battery while it's unplugged and see if anything changes. Do you have any wireless battery powered indoor/outdoor thermometers or anything else that might be emitting any RF energy? Pull their batteries if that's the case. After completely disabling everything, see if the other remotes that are acting up work.

If all that fails, you'll need to call Ghost Busters and have the do a scan!