@Frank looks like you need a trip down to your local Auto Parts Store or stop in at an Auto Electricians shop, and buy some nice 2 core cable - I think 8mm? (thats what I always use on the car) or do you have a "12Volt Shop" near by? or Batteries Plus? they will help you get the right gear you will need - even Jaycar should be able to help you
then run the cable from you Battery to wherever you are wanting to put the plug - through cupboards, or underneath the Van itself - tis not a hard job, just time consuming
how much wire will I need - depends on from where to where - then add maybe 500mm extra
what plugs will I need - the one for external, plus what ever else you want to put in
also need some crimps & crimping tool for the Battery end
if you dont have the tools - could be expensive for the first use - but after that you will be right
as I said - stop into one of those shops and get some ideas from them
cheers