WildeSquirrell safety Socket.

[Bouydog]

I think your point being a one action solution ? plug into supply, all good plug into vehicle job done I realise I have added a separate component and action but for me it was an easy solution that required no electrical wizardry.

 

[wildebus]

I think whilst the "wildesquirrell safety socket" is the ideal solution in terms of convenience, until/unless that is available for sale, Bouydog's combo will work very well and has the benefit over the traditional "plug in and go inside" routine as it:

1. checks without making the connection to the van (a definate advantage) and;
2. saves having to walk around the van to the opposite door and going in to the electrics cupboard & check there is a connection and that it is good.

Currently I have a mains tester that I plug into a socket by the door but that still means doing the walk round to look at it.