Question about making electrical modifications to minivan, conversion van or standard van?

An inverter is very inefficient for a load like a heater,I wouldn't do that.What you are thinking of doing is used in boats especially,it's called using a battery isolator.That is achieved by using a diode [one way valve for electrical current].With that in place you can use a 12 volt heater and drain the battery without affecting the starting battery.A good place for information on this is either a marine or RV store.