Every camper/trailer I've seen has the water filler on the outside. The only problem I see with putting it on the front of the bed is the road grime that will get all over it. Personally, I would put it there and just build a little box with an access door or maybe put in another fuel filler door for it.
Nice work so far, I love this build. Any estimate on what it would have cost had you not come across all that stainless for free?