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

As said, it's not a quirk, and shore power is NOT irrelevant unless your converter is shot.

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?

It is a Good Design feature.

If they didn't work off battery, you couldn't extend or retract them when truly "dry" camping.

Every 5th wheel I ever had used 12v slide motors, which means they work off of the battery. The truck will give the battery power. The converter will give the battery power when plugged into shore power or the generator.