There are many paths to wiring accessories...but never put a fuse holder under the gas tank. Locate it where you have easy access, like under the seat or in the cowl area.
As far as a power harness for the Nuvi, I installed the Garmin bare wire kit over a year ago and I have had no issues...not even a blown fuse. I would recommend using a fuse holder that matches the spare fuses you cary for the other bike systems and I would not use glass fuses on a motorcycle. Don't cheap out on the wiring harness connector, it'll come back to bite you when you don't need a hassle.