If they drop considerably when the slide tries to come in, and they are good batteries, I would try to rig up a direct line with jumper cables to the slide motor.
The most likely source of your problem is a bad electrical connection to the actual motor. If you can troubleshoot, take the negative connection first and see if using a direct line from battery to motor will make it run properly. If not, then try the positive from battery to motor.
But, bad batteries can do the same.. slide would see house batteries only. Engine starting battery should isolate from the house when you are not running the engine unless you have a bi-directional relay.