Two months ago I was having the same issue, thought I had bad batteries. I run Interstates so I went to the Interstate store. They checked their charge and load tested them. One was only charged 60%. I now little about batteries so no sure what it's called, but they also sampled the water in the batteries? Not sure what that told them, but they asked to kept them for the day (gave me loaners). When I got them installed the next day, it was like night and day, they spun the motor faster than I can ever remember. Two months later, still spinning over really fast! Don't know what they did. All for free!
Also, before I took the batteries to be checked I checked the voltage, it was 12.5-12.6 thats good, I have learned volts mean nothing, only amps count, and they must be load tested.