Does anyone actually use an extension cord with their NACS charger, or is that a bad idea?

People do it all the time, and most of the time nothing happens. When something does go wrong, it's almost always at the plug-to-cord connection. That's a friction fit carrying 12A for 10+ hours, and it heats up. A cheap 16 gauge orange cord from the hardware store isn't built for that. The cord itself usually survives fine. The connection is what melts. If you're going to do it, use a 12 gauge cord, as short as you can get away with, with no adapters stacked on top. Keep the connection off wet ground. Check it with your hand after an hour. If it's more than slightly warm, stop. The brand of the mobile connector doesn't really matter for this. The extension cord is the weak link either way.