Been having the same issue since one of the updates in last month. Unplugging with truck on or off does not matter. I been having to select no trailer each time when done towing to get the GOM back to normal reading.
Still could be wiring. May want to have your electrician double check everything it torqued correctly.Both under and over torqued connections can cause overheating. If all to spec then I push back on Ford to get a replacement.
There are two sets of instructions for Lightning splash guard. You were probably looking at the non-molded ones that require drilling previous. Ford has not made any changes to the molded design one.