Look at the specs for your van and find out how many amps your 12v accessory plug is rated for. Many times they are only rated for 10 amps and are pretty much meant for small devices such as charging a phone or powering a GPS for driving directions.
It very well could be that you need to run a new heavier gauge wire (and fuse it) from the battery to a a new 12v socket outlet that is rated for 15 amps. Add a switch for that new, additional, accessory socket that is also rated for 15 amps. Best done from a house battery instead of your starter battery unless you only plan on charging while you are actively driving. you can purchase sockets with mounting brackets from auto parts stores, RV stores, marine, truck stops and of course online.