Your best option to troubleshoot it is to take the power from the battery to the slide motor. When I do something like this, I use a long jumper cable. If the motor runs when you connect assured power to the motor leads, then just find the connection points in between and work your way back.
If the motor does not run, you have to figure out why.
Let the battery sit unused and disconnected for an hour, then put your meter on it. Then turn on some fantastic fans, exhaust fans, and lights and check the meter. Your voltage should not drop very much at all if the battery is good. If it drops below 12 volts, your battery is toast. It should only drop about 0.2 or max 0.3 volts with a big load.