So, there may already be something on this somewhere, so happy for this to get deleted, but I thought I'd relate something I found interesting in the hope it might help someone else.

My truck has never lost sync. Unlike some people's who seem to go through the issue regularly.

When I say never has ..... what I mean is hadn't until last night. Jump in, hit the key, nothing. Check it is in park, still nothing. Cycle the gears to check the XYZ, still nothing, then I noticed no CEL. Ah! loss of sync. cycle the key a few times, CEL back on, starts up and off I go.

I was pondering later, why, after two years of trouble free starting, would this have happened. What did I do different. So here it is for what it is worth.

I have an aux socket in the back. When I wired it in I took the feed directly off the under seat fuse box live feed. This feed, I assume, is where the BeCM gets is power from. So what did I do different? I plugged in a compressor driven fridge to the aux socket just before I went to start the truck.

Did the consequent lowering of voltage at this feed point interfere with the BeCM and its communication with the ECU?

Are the loss of sync issues come people have due to a poor connection between the under bonnet supply and the BeCM supply, resulting in a dip in voltage when you turn on the key?

No idea, but I thought I'd throw it out there.

Of course my experience might be totally unrelated at all, but .......

Could be you are right, or interference induced into the power line corrupts something, maybe try ferrites on the fridge power cables? I've had mine do the same, turn the key, nothing, check the gear lever, in Park but cycled it anyway, tried again and fired up without problem. Never noticed if the CEL is on or not though, it isn't the sort of thing you look at. Odd thing is, cycling the key won't cause it to send a sync code so if it was out of sync the first time it still will be. Maybe Relay 15 or 19 is getting tired and not immediately operating so the engine ECU isn't getting power?

Relay 15 not operating will give no CEL and also no ABS pump running. If it does it again, check if that is running which might give a clue.

Not quite the same thing, but when I wired my auxilliary towing socket (the white 7 pin 12s one in the UK), I took it straight from the battery with heavy duty cable. Just a fuse in the line. No switch or anything.
It powers the caravan fridge via a relay in the caravan which only energises when the engine runs. I have not had any problems with sync due to that. Never needed ferrite sleeves either. It is an absorbtion type fridge 12v/240v/gas.
A compressor type fridge will take a fair amount of current on startup. I would be reluctant to take a heavy feed from the BECM. Better direct from the battery imho.

Absorption fridges are simply a heating element, so no particular surge, and no interference.
They do draw more current than an equivalent compressor fridge. Around 12 amps if memory serves.
So, Dave, your method would be much preferred as it leaves the stock wiring stock.
I have noticed with both my Classic, and the P-38 that RR used the barest minimum wire size possible....
Probably to cut costs. This basically means you cannot just hook up accessories to any lead you want to as
for the most part, they are already maxed out.

Compressor fridges have a controller that soft starts the compressor motor, so no real surge there.
They DO however throw out a fair amount of RF hash on the DC lines (as well as in the airwaves)
I have ferrite cores on the twisted pair shielded power lines on the 2 fridges I have onboard the boat here.
It is certainly possible it was interference from the compressor controller glitching the Becm.
If you think it was due to power draw, then simply connect a heavy lead (Fused) to the same wire, but at the bottom of the fusebox rather than at the Becm end. This would eliminate any voltage drop in the cable to the Becm.