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

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!