[scottz]

Your tech was correct, the slides (and other things) run off the 12V batteries. The converter, when supplied with shore power, charges the batteries.

Think of your batteries as a pitcher of water or the water behind a dam. The water (voltage) is the potential used to power things. As you run DC devices (including slides), the level gets lower. The converter (or your truck while driving or solar panels if you have them) refills the pitcher (current); sometimes it can't keep up.