Paralleling car batteries
You can buy split charge relays or controllers that come in many different types and sizes. 

You can buy a simple split charge relay that has voltage sensing circuitry that charges both batteries and will keep both batteries separate so the starter battery never gets discharged below 12v. These can be found as cheap as £/$10

You can buy more sophisticated micro-controlled systems that monitor both batteries and charges each and switches between the two, they also often have inputs and outputs for extra charging sources such as extra alternators and solar panels. The can also have dedicated outputs for critical equipment such as fridges, communication/GPS devices etc. These range in price from £/$50-1000 depending on the spec.

You could also use a DC to DC charger to maintain the second battery this would be required in some modern vehicles with 'smart alternators'.