Trucharge 40+

I have a truecharge 40+ about 12 years old & lately the max charge rate is 30 amps according to the remote guage.My 6 volt batteries (2) are about 5 1/2 years old & I have a battery disconnect that I can use to charge the engine battery if needed. When charging either battery or both when they were about 50% discharged the max amps is 30 amps (I know it splits the charge when 2 batteries are charged)I don't see a big difference in charging time, but it use to start at 40 amps on the same 6 v. batteries. Is it the batteries? The engine battery is new. Thanks for any help.