Here's some more info on my experiences. Hope it can help someone figure out why they are getting this alert.
Disclaimer: I'm no expert on trailer or Tesla electronics or wiring issues. Just spent a lot of time reading up on things to try to understand what's going on with my rig.
I've had my Model Y back to the Service Center three times to troubleshoot this alert.
Here's what I've learned and what I plan to do as a work-around to avoid the problem:
It does seem consistent that it happens after the car has been shutdown for a while with the trailer attached.
The brake controller, also a Tekonsha P3 in my case, and the brake controller wiring have been exonerated. The problem occurs even when the brake controller is disconnected.
The SC, I think has done all it can. They replaced the VCFRONT body controller with no effect, and have done extensive checks of the wiring and the logs. They even tested it with a trailer they had at the SC, but couldn't recreate the problem.
They did convince me that the DC-DC converter in the car and the car's 12v battery are not likely to be at fault.
The problem seems to be an overcurrent that trips an Efuse in the Tesla, in my case, while the car is shutdown or when it is waking up. The Efuse trip is detected during self-test when the car wakes up, and the alert is presented. The expanded alert, according to the SC writeup is VCFRONT_a552_eFuseSelfTestFailure
The Efuse apparently protects a number of things and is not specific to a particular wire in the 7-pin connector. It is tripped by an overcurrent which could be a quick rush of current either too or from the trailer. There are no diodes in the wiring to the trailer, so the current could come from the trailer's battery, or likewise, there could be a drain going to the trailer's battery, or the lights.
I haven't gotten a clear answer on what amount of current will trip the Efuse, but have the impression is might be well under 15 amps. It is my understanding that the Efuse trips much faster than a conventional fuse or circuit breaker, so it might be a very short rush of current that might not affect other tow vehicles.
Overall, my impression is that the Tesla is sensitive to current fluctuations that might not affect other tow vehicles. I think the most likely source of the fluctuations in my case are the trailer battery having a higher or lower charge state and either backfeeding the Tesla or causing a drain exceeding the threshold for the Efuse. Note: On my trailer the battery is wired directly thru to the power pin (Battery power) in the 7-pin connector.
I've never had the problem occur while driving, over 3500 miles with the same trailer, just while completely shutdown with the trailer still attached.
The SC suggested that when I shutdown, I disconnect the electrical cable to the trailer, which seems reasonable as long as I remember to always connect it again before continuing my trip!
It seems likely to me that the trailer's battery is what the Tesla doesn't like. The lighting could have some kind of issue, but I'm doubtful of that. All the trailer lighting is checked before each trip and all are working fine. Also, I don't have anything internal, like a fridge, running on 12v on the trailer while it is connected to the car.
My plan is to pull the inline fuse on the battery + wire when I shutdown with the car still attached, and stick it back in before continuing. This effectively disconnects the trailer battery from the car, and the trailer.
I'm also considering installing a diode on the Battery+ lead of the 7-pin connector cable to prevent any current from flowing back into the car from the battery and see if I can get away with leaving the battery fuse installed while the car is shutdown.
To be continued...