Step up to an installed charger like Guest /Marinco and maintain your batteries properly though out the entire year.
Another option either installed or mobile is the CTEK 7002 or others in their line. http://smartercharger.com/battery-chargers/
More advanced charging and desulfation capabilities.
Battery Tender / trickle chargers are old technology and low power.
Batteries are a big investment and while something is better than nothing, there are better products that can significantly increase battery life.