They make a plug that is just the male portion that is slightly recessed to mount on a vehicle. Lots of us diesel guys have them for our trucks, so it's easier to plug in the block heater. You may also be able to drill a small hole, cut your extension cord, poke it through the hole, then resolder everything back up. That way you could pull the cord out when you needed it and pull it back into the trailer when you didn't (with only the plug poking out). I'll try to find a part number for the truck plug. One of my buddies went even more low budget and took a recessed cover, cut the slots in it for the plug prongs and glued the plug to the backside. Then he plugs the extension cord to it when needed. Of course, if you're riding even weekly, don't worry about it. The worst you'd have to do is replace the battery a year or two earlier.