second battery to charge off the engine like the original?

It should be very easy to wire in a second battery. Some vans and trucks come with a circuit for this pupose.
Make and model of van would be helpful.
Problem is locating the battery so it is vented to the outside as it is not good to have battery gases inside living areas. Yes, you could get sealed batteries such as AGMs but they are costly.
Some rigs have enough room to put under the hood oppoite of the existing battery. You have to look at your rig.