Do any of these systems use a "battery isolator"? It's function is to sense the charging load on the batteries and switch the charging to the weaker battery when the other one is fully charged. Takes a TON of load off the alternator as far as trying to maintaing a good charging amperage to keep multiple batteries juiced.

I went with a dual battery setup in my VW (ran the stereo independently on one of the batteries, while the rest of the car electronics ran off the other). The stock 55-amp Motorola was ill-suited for maintaining a charge to two batteries. The isolator allowed the alternator to maintain the charging load, and I wasn't faced with replacing an alternator every year!

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car