Advice please - again.

Yes, I would start by checking all of the fuses.

If that doesn't show up any problems, the best thing to do (IMO) on a 2023 van is to take it back to the supplying dealer for them to address under warranty. That way they should (in theory) fix it in a way that means it won't keep happening.

Allan,
As Roopert mentioned, on a ‘23 van, dealer visit/call definitely, you said RAC called, I’ll assume via Fiat breakdown, either way hopefully you got a receipt for the battery as it may help later. Over weekend, if it’s 2023 and has Lithium leisure fitted you could check the Sargent DC-DC charger is charging the leisure battery while engine running(or if not lithium the split charge system should be charging the leisure the same while engine running) that would at least prove the alternator is supplying charge, and therefore should charge van battery-or possibly point to fault between alternator and van battery.
The solar as you are probably aware can at least be checked again just by selecting the leisure to prove that working at least from the panel through the controller and Sargent system to the leisure, that would at least again narrow down fault forward to the van battery. 
You should see the van battery with a higher voltage at least just after engine running/driving again to help prove alternator ok(even with smart alternator after a run, or put some load on, lights etc to get it to generate charging voltage as such)
The receipt may just help either way if dealer pushes you toward Fiat for warranty/fault checking
Cheers Sheppy