Issue, only when towing the horse trailer
"Lamp Out" and chime will come and go, and come and go and ..............
straight level road; "Lamp Out" usually clears.
turn left or right, pot hole, bumps, ....... "Lamp Out" and chime will come and go, and come and go and ..............
all the lights on trailer are working. and don't flicker while this is happening.
turn off lights on truck, "Lamp Out" clears.
neither brakes lights nor trailer brakes nor turn signals cause issue (those work fine)
trailer plug-in is tight.
trailer has a mix of LED and incandescent bulbs

What is the solution?

wish there was a switch that could tell the computer there is a trailer back there and ignore the lamp sensor.

 

There is your problem. Switch 'em all out to incandescent. Or all LED. The problem will either go away, or, be present whenever the trailer is plugged in. I suspect that if you go all LED, the problem will be permanent.......

 

update
replaced plug-in on truck, blade broken
lamp out went away for a couple days

now 'lamp out' is displayed when lights are off (all lights) and trailer unhitched

when lights on, nothing is amiss- lights are lit 'lamp out' is displayed
when lights off 'lamp out' is displayed

GOOD NEWS chime stopped! issue must have finally locked the program into alarm