Not a quirk, all RV slides are on the 12V system. That's not necessarily just from the batteries, as your 120V to 12V converter contributes some power to the task if you are plugged in, but might not be enough to do the job alone, depending on converter size and how much power the slideouts need.
Power from tow vehicle connections is always less, this has to do with wire sizes and connector losses. In a motorhome, with the engine running, power from the alternator might be the strongest source for moving slideouts, but in a towable RV the strongest source is usually the storage batteries, which should be fully charged by the tow vehicle on the way to the camp, or by the converter before you leave.
