My 2010 4.4 Tdv8 L322 was put to use collecting a P38 for a friend yesterday.
He hired a nice newish Brian James Trailer, however the turn signal on the dash wasn’t show a trailer connected (assumed it’s because the trailer has LED lights so not enough load).

Driving for an hr with the trailer, turned engine off to load the P38 and 20min later my L322 would not even turn over (did have dash lights and it measured a battery voltage of 12v showing on gap IID)
Managed to get it to start from a jump and fired up no problem.

Got back to mates place after another hrs driving, removed trailer and then another 45min drive home, turned off and measured the battery voltage at 12.6V , Gap IID showed it as charging at 14V (multimeter said 14.3V) .

Left it overnight and fired up this morning no issues ..

Gap IID shows fault codes and faults that I believe are standard L322 quirks,
Stuff related to the audio amp speakers, lost communication with handbrake, door lock open circuit, left seat cushion fault and open circuit…(apart from the heated seat, all working fine)..

But it also shows turn signal B1D06-15 “Left turn signal general failure- circuit short to battery or open” and this was logged just before I turned the engine off to load the P38 onto the trailer .

Does anyone know if a faulty trailer was connected, could this cause this code and possibly a very heavy drain ?

As i’m getting 14.3V when engine running, I don’t think the alternator is at fault, i’m just reluctant to put a £200 battery on it, if the battery is not at fault and it was the trailer causing the problem.
Equally don’t want to play “will it start” roulette if others have experienced the above and it’s more likely to be a failing battery.

Has anyone experienced similar, a battery failure, then fine the following day ?

I havn't got any codes showing but my car has done a similar thing with my caravan. Had a bit of a break on a journey and just like yours I had dash lights but not a hint of it trying to start.
I checked all fuses and relays and found nothing wrong.
I tried unplugging the caravan electrics from the car and it fired straight up. It's done this twice now and I have no clue why.

I did try unplugging the trailer, but it seems like what ever had cause the battery drain, had already pulled the battery too low to start it .

It’s like having the trailer on, somehow stopped the battery charging.

I should add, i know my tow electrics didn’t have any faults, I towed my Caravan a few weeks ago without any issues.