Renogy LiPo sudden discharged

[Bat21]

You could remove the battery from the van and sit it in a plastic storage container with a mixture of ice cubes and water, this will stabilise at 0 degrees eventually as long keep solid ice in the mixture.
Keep the water level just below the top, connect a multimeter and see what happens.

I could remove it from the truck and stick it in the fridge which should drop it just below the 5c heat pad threshold. If I leave the Bluetooth module plugged into it I can monitor the temp and current drain and voltage.
I'll see how it goes over the next couple of weeks first.