Slides are powered by 12 volts. When plugged in, the converter should be charging the battery and also should be able to supply enough power to the slide motor. Battery may indeed be bad pulling everything down. Put a voltmeter across the battery as you run the slide.
Retired electronic technician (45 years in the field)
2017 Greyhawk 29W
solar & many other mods
wife (maybe I should have given her top billing)