Lost connection to Van Battery!

[NikonP]

Hi. I have Fiat Ilusion (Ducato) 2023. The main switch 'lost" connection to van battery. It showing 0.0v. I know that it's running currently at 16V. So it's beeping and the fridge etc is not supported while driving. Leasure battery is fine.

Any idea where should I look for a fault? I've recorded short video:

 

[Speve]

Have you checked the main earth strap to your starter battery it could be a poor connection