New member in So Cal with charging concern/question

Hello all,

I purchased a 2003 GEM 4 seat car from a lawyer in Camarillo, CA. She had put 6 new Costco 27 group RV/marine batteries in the car. It also has a Delta-q charger that was added at some point before she had owned the car. The batteries test okay but they don’t seem to hold the charge for long. I have charged all six of them individually on separate battery chargers and even though they test at 12.9 volts they dissipate quickly. I never run them below 55% of charge and hook them up to the onboard Delta after usage. I don’t know if this is considered normal behavior for undersized Costco batteries or if there is a charging issue. Any suggestions?