I truly appreciate all the insight. The manufacturer of the slide motor feels confident that the motor is the problem; he said for them to start to wear out after 11 years is quite predictable, especially the one on the heaviest slide. I'll continue to monitor the batteries and change them when they obviously need it.
I just got off the phone with the distributor. $620 later, a new motor is enroute. And it's more powerful than the old one, with more torque, so maybe this'll be the fix.
Thanks again y'all! That's why I love this forum, and point as many newbies to it as I can.
dave