1. Power your kit from a small bank of sealed automotive batteries (gel or AGM) which can be mounted in any orientation and safer than traditional wet batteries. If you plan on pulling power from them for extended periods without recharging (eg gaming all night) consider deep cycle batteries. Build secure mounts for them in the back of your van so they don't move around and have some protection in case of an accident..
2. Charge these batteries with a high output alternator driven from the driveshaft (do a web search on "driveshaft driven alternator"). This will only charge your batteries while the van is moving, but I do assume you're planning on driving it some of the time.
3. If you don't know how to hook up the batteries in parallel, or attach them to the alternator, or how to correctly size, crimp, and route the wiring find someone who does and do it right. Screwing up here could make things a little more intense than you'd like.
Doing it this way has a number of advantages. The two big ones are:
1. The system is completely isolated from vehicle electrics so, for example, completely draining the rear batteries won't keep you from starting the van.
2. Having the batteries and alternator both at the rear of the van makes the wiring easier.