Re: engine batteries-short life??
Reply #1 –
The first thing I would suspect would be the series of connections between the starting battery bank and the engine's starter. Corrosion can seriously degrade the quality of these connections. Starting with the connections to the batteries themselves, make sure everything is tight and corrosion free.
Second suspect would be an older battery charger that is giving the batteries a "trickle charge" instead of a "float charge". The old converter/chargers were seriously destructive to batteries. If you are still using the original consider upgrading to a 3-state Progressive Industries (or equal) version.
If everything is ok with those two then it's either the batteries, the cables themselves, the isolator, or.... well... I dunno.
Good luck. 
Craig