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I would wire it straight to the battery, just be sure to include an inline fuse in the positive lead. If you are not wedded to a cigarette lighter style connector then look at "SAE" type, they are flatter and physically smaller, thus easier to store. There are SAE to USB adapters made by Battery Tender Inc that will allow you to charge your phone etc.
It would be prudent to use a relay to switch on and off the power to your Power outlet with the bike's ignition. This way, you cannot come back to a bike with a dead battery. Wire up the circuit as has been described previously, however, introduce an 30A automotive relay in series with the power lead just after the inline fuse. The wires will connect with pins 30 and 87. Wire pin 85 to ground and pin 86 to a lead on the bike that gets energized when the ignition turns on and disabled when the ignition is turned off. This type of setup will not put undue load on the existing circuits and you can be assured of not disturbing the existing fuses.