Wal-Mart carries accessory 12V cigarette lighters in the electronics section beside automotive.
Look around the same area where they sell the volt meters.
Just two wires; one hot and one ground. Run the wires under the tank, which is only 2 bolts and 5 minutes to remove. You may have to extend the length of the wires. I'm not sure. But it shouldn't be a problem.
(Snip off the battery clamps and splice in your own prong connector directly to the battery. Your ground can go anywhere, really.) They even make little plastic covers, or lids, so junk doesn't get inside your outlet if you have it outside. Might have to buy that separate. Good luck!
Rig it up and splash some photos up in here!