Thanks for the tip. It is a micro USB, but I'm gonna run the cable up through the bottom of the tankbag and plug it into the tablet; which is under the waterproof plastic. So in theory, I should be alright.
I will probably just run it to the battery directly. If I am understanding you guys, as long as nothing is plugged into the power supply it is just sitting there dormant not draining the battery. Well, the only time something will be plugged in is if I'm riding it, so I will just keep it simple and hook it directly to the juicebox.