Low tech solution for a once-in-a-while need for aux battery:
I installed a cigarette lighter style receptacle on the side of the driver seat base by the door; it is switched via relay to only be on when the key is on. I plug an 800 watt inverter into it, and a small 2-5 amp battery charger into the inverter to charge an extra car battery that I have (sealed L/A). When I want to use the juice from the extra battery while parked, I attach the same inverter via clips to the posts of the extra battery and I have 110vac for charging batteries, powering laptop or running my PA system (rare). I made a small plywood box that houses all this stuff and sits behind the driver seat or in a cabinet if we're in camping mode. I also run my thermoelectric refridgerator off this switched 12vdc power receptacle.
I usually just charge up my batteries while driving and leave the car battery at home. I'll charge powertool and AA/AAA rechargeable batteries, and I've found that re-charging them with the motor off (from the dash receptacle, always "on") produces negligible drain on the battery. My AA/AAA recharger plugs directly into the lighter socket (I attached a dual socket to the bottom of the "cup holder" that is wired to the main socket, so I can plug in phone, GPS and battery charger at the same time).
At some point I'll invest in an isolator and have a true aux battery, but I don't need one very often.