Whatever you do it will take up space, I would build a wall behind the driver and passenger seat with a small door to get to the cab. Both sides of those walls provide space for things like charge controller and inverter. The battery can be in a wood box that sits on the floor and doubles as a seat. Maybe one on both sides. A lot of van builds also put all that stuff right under the bed, if it's a van they plan to sleep in.
Also have to consider how long people would actually be gaming for at night. Let's say you do have a gaming session going on, and everybody works the next day. You might game for like 1-2 hours max. It's not like you're gaming all night. You just need enough battery to last a couple hours. If by chance you game longer because it's a weekend and you plan to do an all nighter, then you can just start the engine or hookup a generator or to shore power once in a while to top up the battery. Just make sure your electrical is setup that way where you can plug in an AC source, but also that it can charge from the vehicle alternator. Gives multiple options.