I would replace the 6-year old T-105's without further ado. Then, I would run both banks as one single bank, giving you 450AH @ 20hr rate.
When fully charged and rested overnite with no load and no charge, you should measure 12.6-12.7V at the battery terminals (the only way to measure accurately in a new-to-you boat). Use a good digital multimeter, calibrated if possible.
For a new smart charger, I'd recommend you consider the Iota DLS series. They are IMHO the best value on the market today...well built, smart, reliable, and with a PWM charging technology which helps your batteries to last longer. They are a bargain at the discount prices offered by, e.g., Arizona Wind Sun. For your 450AH bank, I'd recommend the 75A model (DLS-75/IQ4).
New batteries need to be exercised a few times times (med-deep discharge followed by full charge) in order to reach their full capacity potential. If you have reason to believe they've been sitting awhile at the dealer's, it might be a good idea to equalize them, too.
Last edited by btrayfors; 07-23-2009 at 10:51 PM.