You might want to consider jumping up a wire size due to the longer wire run, you need to account for losses across that distance. As far as runnign wires under the deck, it is not that difficult, there are wire runs you can access from any of the hatches in the deck, from where your batteries are now, and from the wire chase locates somewhere below you helm with access in that general area, likely covered by removable panels. The use of a 6ft wire puller will help a lot, using a wire snake would be much more difficult, the wire puller will do it in a few minutes tops, even quicker if you have someone helping. Tracking where the wires need to go first will take longer. Taking a peak at your owners manual will show all your wire chases and wire tubes and give you a good idea of how to chase your route. Personally I'd avoid running the wire itself over the aluminum tank, run it around it and avoid contact as a precaution incase it ever chafes, if possible, use the wire tubes for the run to make the run safer.
