Power a device from 12v adapter instead of ac wall adapter
You just need to find the right barrel connector for your clock and wire it to your camper 12V battery. If this will be a permanent installation, wire it directly to a 12V tap. If needed temporarily, wire it to a cigarette lighter adapter.

Take your barrel connector to RadioShack and you should be able to find it there. You may also pick up some wire and cigarette lighter adapter.

Make sure to watch for polarity when you build the adapter.