when you cut the hole for the box - I would make a template out of cardboard and make sure it is a tight fit around the box before cutting the camper.
a Dremel tool works great for making the hole - just use the reinforced discs if you go that route.
I would use Dicor Lap Sealant around the box to seal it to the camper
and like stated above - I would try to keep it in a area where it wouldnt be hard to run water lines to it so it makes the install smooth and easy
Here is an Install I did with adding a Locking Door to my fresh water tank - pictures and all so it gives you an idea of what I did to cut the hole and seal it - Locking Fresh Water Door Installed - Mini Lite 2306