Cody - What you are looking to do is standard in my Roadtrek. There is a 750 watt inverter/converter/charger installed that manages all electric in the B. There is an engine battery and an AGM coach battery (installed outside the van - I have two AGM coach batteries). There is an isolator installed between the engine battery and the coach battery so that neither system drains the other. Both the coach battery and the engine battery charge when driving and also when plugged into shore power (plus in mine when running the on board generator). Take a look at the Tripp-Lite 3-way units to install with your battery to take care of charging. You will not get a lot of time out of the battery between charges if you run too much on it or for too long.
