Depending on how you van batteries are wired or placed you might need two master switches.
All you need to do is interrupt the ground wire to each battery bank.
You would need something like one or two of these.
http://www.amazon.com/s/ref=nb_sb_ss_i_ ... Caps%2C661
You could just disconect the batteries when you leave it for any length of time.
You could also add solar and or a charger (the batteries should still be charged after sitting disconected ) and keep the batteries charged instead of disconecting them
That said if you want to keep radio memory and clock that would have to be rewired.
If you want the battery switch to be more convenient, like inside the van then the job goes from a somewhat simple job to a much bigger one. You need large sized wire and terminals plus labor for running the wire and such.
I am not sure where you live but most places I would think charge from $75-125 an hour for something like that. Add a few hundred or more for parts and that $800 goes quick.
I would just buy a smart charger at least and go that way just plug it in and leave it when you are not using it. You could get on board mounted ones for each battery bank or just buy one portable one for all of your battery charging needs.
Hope that helps...