Transit custom total power loss.

Hi guys, been searching online for an answer to this and haven't been able to find an exact answer to the issue I've had. 

I was recently on a journey in my van, had been driving fine for 2hrs, stopped for a short period of time and couldn't start again, almost like a dead battery. There had been no problem starting previously. Eventually got it started again by disconnecting and reconnecting the battery terminals. 

Now, I assumed the issue could be the fact I had just installed a 12v charger in the rear of the van as the wiring was already there for it and other specs have it installed. I was running a 12v cool box off this at the time, I disconnected this and haven't had the issue since. Battery still fine, checked voltage etc. I'm now assuming the van detected the 12v socket as a drain and shut down because of it? I've checked the wiring at the socket and all seems fine. Thinking about replacing the socket and trying again but I'm not sure it'll help. Could also be the box itself I guess but that was working fine.

I'd appreciate any ideas if anyone has had similar? Like I say, if I'm not using it it's completely fine and even if I do it will work fine for a good few days/weeks before it shuts down. 

I have a single battery. 

Thanks