I invested in two Trojan 6V deep cycle batteries which weren't cheap, and I knew I didn't trust my factory WFCO converter to take good care of them for the reasons you have mentioned.
I bought a Progressive Dynamics smart charger which was a direct replacement of my original one, using the same housing (just different guts). It gives the batteries exactly what they need, when they need it, and allows manual override if you're looking for a rapid boost charge. It drops down to 13.2V for storage mode as to not boil or over charge, and every day it bumps up to 14.5V for 20min I think to desulphate the batteries.
Not too expensive and other than checking water levels a couple times a season, it takes care of itself.
4600 Series Upgrade or Replacement Power Converters