Welcome to the forum.
The converter may have its reverse polarity fuses blown, but you should be able to run the slide once the RP is corrected from the battery alone.
Yes, the Lippert electric slide works from its own pos wire on the battery (often red) with a 30a fuse (check that too) separate from the other (often red) wire for the DC fuse panel-to /from the converter (with another fuse for that wire near the battery)
The slide motor does work in reverse polarity at the IN/OUT switch. That switch is grounded to the frame as is the motor, so pos wire to battery runs that. RP at the battery would make that wire to the switch neg, but not sure it would make the RV frame positive.
On the battery red and white wires--red is pos , white is minus. If red and black-- red is pos, black minus. If black and white-- black is pos and white is minus.
Other colours, use your voltmeter--if it shows a - in front of the numbers, that is an RP, and if it shows just the numbers, that is good.