Yeah, the starter is a good bet. They wear out so slowly its hard to notice. I used 0 gauge for the cables. With the two batteries it just spins right to life. I'll try to get a video the next time its cold and light out at the same time.What size cables did you use? I'm still having cold starting problems out of mine. When it was in the Dodge, I actually drove it for about a year starting it with the battery out of my Honda Civic. It started the same as with a full size battery. I just swapped it in anytime I needed the truck.
I'm currently running new cables from an auto parts store. Same as I had in the Dodge. It's got to be the starter.
xs-power d3100
Delo 5w40 synthetic
cables are 2/0.