My suggestion is not to move the water tank, certainly not to add tools.
Ginny uses the center storage compartment under the couch for two bins of her clothing. Behind those bins I have built a shelf that carries most of my tools.
I do also keep a drill and sockets near the water tank. There are lots of little places to store stuff.
Beside the couch's center compartment. towards the front door, I cut the fiberglass and installed a large drawer for more of Ginny's clothes, probably 18" wide and 18" deep. The front area of the Scamp, behind the drawer is curved. I built a little storage area behind that drawer. I keep electrical
items there, a meter soldering iron, wire....
Here's one I put two drawers under the non-tank dinette. The drawer are on slides, under one drawer is my supply of sand paper, a little space normally unused. Behind the one drawer I have a wine rack (stand-up). Under that same dinette, I have a bean pot, a crock pot, a cube heater and a lantern.
In 15 years probably my best tool is a bottle jack I carry in the tow vehicle so I have it when out exploring.
As to moving the water tank, we had a trailer with a rather thin gray tank mounted under the trailer attached to the floor, about half the thickness of the Scamp's grey tank. maybe the same thickness as the bottom of the axle to the floor. A water tank could probably be mounted under the trailer (though I haven't looked). The tank had a low point so it drained well but slowly. The pump would fix the slowly. We protected our grey tank with an inch of blue foam glued on. The purpose was to protect it from stone hits on the Labrador highway but it would also insulate.
You'll see I have two plastic container attached to the floor in front of our bumper box. Each of these plastic boxes is not much smaller than the Scamp's Water tank.
Like most people I'm not a fan of mounting weight on the bumper. The only time I fill our tank to full is when entering a boondocking
site. Typically we drive with a half tank.
Hope this helps.