Something else is wrong. The slides should work on shore power and battery power without being connected to a truck.
I suspect your converter may not be working. Depending on what type of converter you have there may be fuses on the converter that have blown. As already suggested, you should get a voltmeter. Check the battery voltage while on shore power. You should read 12.7 volts. If less than that, the converter is probably not working. Also might want to have the battery tested.
if you run the battery down enough, things like lights will still work, but high current draw device like a slide motor will not.