? Slides -- Design flaw or "the way they all do it"?

Not sure why you think that's a "design flaw". Of course they have to run off 12vdc since you don't always have 120VAC available.

When on shore power, or generator power, ALL of your 12vdc components (including the slides) are actually operating from the converter rather than just the batteries. When hooked up to the tow vehicle, it's alternator is charging the batteries.

Again, how else would you suggest that the slides operate?