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

12v makes a lot of sense as many camp away from shore power.

Running it thru the battery avoids overloading the charging system...particularly the truck plug doesn't provide a ton of amps. By running it thru the battery, the battery can easily handle the amperage called for (obviously from battery to slide motor needs sufficient cabling but the manufacturer is well aware of that)