I was fishing and went to crank my boat up to head in and it turned over really slow. It seemed to me like the battery was almost dead so I swapped with my trolling motor battery and had the same result. When I got home I tested the voltage at the starter and it was dropping from 12.6 to around 8 volts. I replaced the starter and am having the exact same issues. I have load tested the batteries and both are good. What else should I do?