How far is it from the head to v-berth?
If the head is in the aft end of the cabin, it's prob'ly too long a hose run from the head to a tank under the v-berth, but if it's not longer than a 6-8' run, that's the ideal place for it. There are several articles in the Head Mistress forum Reference Library (on the forum homepage) that should help you figure out how to plumb it, and what size tank you need for your boat and use.