Battery life depends completely on how much you draw on it. Keep the lights off when you don't need them (and convert them to LED bulbs when you can), the same goes for anything else 12v electric - the less you use the longer the battery will provide power. After a time you will get the hang of how long you can run things on a single battery and decide if you need/want an upgrade. True deep cycle batteries will give more 'run time', multiple batteries will give still more, but you have to consider whether you need more or not, whether the cost + weight are worth it, etc.
I would also get a multi-meter to check the battery. If you prevent the battery from dropping below 12v it will last a lot longer and perform better over it's lifetime. As a battery is used it loses capacity, even when properly cared for - the more deeply it is discharged the faster it will lose capacity, and if you really drain it hard it can 'die' quickly and need to be replaced. Most batteries provided with your camper at time of purchase are not that great (about 50 amp hours usable, perhaps less), so be careful the first time out and monitor the battery voltage.
The table in mine was under one of the bench seats, but the dealer went though everything with me at the time of pickup and made sure that I was happy with everything, that everything was there, etc. If you don't have the other items you might want to check with the dealer to be sure that they were not removed, or are hiding in some storage compartment or under a cushion somewhere.
1975 Westfalia (VW)
2014 Rockwood A122