Wiring set-up for battery switch.
I''m no expert at this either, but I doubt the battery is the problem if you can start the engine by jumping the starter. Sounds more like a starter/solenoid problem. (This from a liberal arts major.)
Do buy a digital multi-meter and check the charge level of the batteries. As a guide, at 80 degrees F, a lead acid battery will show around 12.73 volts when 100 percent charged. The volts drop to about 12.1 when 50 percent charged. You don''t want the battery to drop below 50 percent charge for very long, and expect the battery to last. In other words, you aren''t dealing with a wide range of available voltage in a battery to start with. That fact alone will save you some grief down the road. You also can get a decent charger for about $100 that won''t fry your batteries. Overcharging them is as bad as undercharging.
I used to get 2 years out of a battery and think I was doing well. Now I get double that, basically because I have learned more about the care and feeding of batteries -- at least enough to be dangerous.