That sure sounds like a dead cell in your SLA battery. The thing is that you should usually get more than one year out of the battery unless it is in a hostile environment. Temperature extremes? Unregulated charging above 14.5 volts? Your DC might read low without a good battery if your trickle supply isn't filtered. I guess you could keep a 1000uF, >= 25 volt electrolytic cap in the truck. If a large filter cap on the output provides the proper voltage, a new battery is probably all that you need.
Joe
