Use a continuous-duty solenoid to connect the auxiliary battery to the charging system while the vehicle is running, using the ignition switch to power the relay. With the ignition off, you will have a way to draw power from the extra battery only, with no drain on the starting battery. Most RV dealers will have that solenoid, but they're also available on Ebay without paying the RV store ripoff prices. Those guys are as bad as (or worse) than the old truck resto-ripoff parts guys!
Search "battery isolation relay" on Evil-bay!
Jerry
