Whats the best way to maintain a 12v SLA battery in an automobile?
Would a direct 12v connection to the vehicles battery system be sufficient, or is there a circuit that should be placed in between for maximum health retention?
First of all, SLA is not the optimal kind of battery for starting an internal combustion engine. SLA are optimized for deep discharge, but their output current has been sacrificed. You need a starter battery (like the first one that's already in the vehicle). Starter batteries are designed for high current output (the cold cranking amps).
Dual battery systems are fairly common on: boats, RVs/caravans, emergency vehicles 1. There is usually a battery isolator, which prevents the secondary battery from draining the starter battery and vise-versa. There are 2 classes of battery isolators:
Either kind of battery isolator would work for your case. Relay-based is more convenient, I think.
In your case where the second battery is used as a backup starter battery, you can disconnect the backup battery simply with a manual switch. Normally, the backup battery is disconnected2. Connect it to the alternator for a couple of hours once every so often to top it off.
The charging current doesn't require a hefty wire, but the starting current does. So, either provide the hefty wire, or carry the backup battery closer to the starter and use jumper wires, when the main battery is dead and it's time to put the backup to use.
1 In these cases, however, the second battery is for powering appliances. In those cases, the battery is indeed an SLA.
2 If it's not disconnected, it's not a backup. tm