Help with external socket identification

Hi everyone

We have three external sockets on our van. 
1. 230v input

2. three pin 230v output

3. unknown!

The unknown one has a black plastic pipe (c2.5cm dia and about 5cm protruding) with a hole down the centre of the pipe and a cut away making the hole more like the shape of a rudimentary key. It’s covered by a black rubber cap and the whole thing is behind the usual white flap. 
We thought it might be external gas, but it looks nothing like others we’ve seen. 
I’ve tried to upload a pic, but can’t get down to the (tiny!) maximum 500k file size. 

We’re probably being daft, but does anyone know what this is?

cheers

stu