We carry 50gal fresh, 15 gal black, and 30+or so gal grey capacity.
The grey is an added on tank, not a factory item.
This is enough for 2-3 days dry camping if you're conservative.
40 gal fresh is definitely not overkill, in my opinion. You won't have to fill it, if you don't want it, but it's nice to have if you need it.
15gal over floor, toilet on top works great.
Grey: 30+gal under floor works very well also. Grey tank on top of floor is not adviseable, need pumps and buffer tank for proper shower function.
Get a flat tank and mount it under the floor, close to the black tank. Makes plumbing easy, and does not require pumps/engineering to work.
The demand pump will come on as soon as you open the tap. It works on a pressure sensor in the output line. Get a pump/accumulator combination, it smoothes out the flow and keeps things quiet.
Try and get all the tanks as close to the axles as possible, and keep them as low as possible for better handling, generally.
If you drop the belly behind the axle, and take a look, you will quickly see if there is room. You can mount it between the frame rails, and between stringers. If it needs to hang low, then just simply fabricate a belly pan piece to go around it. It's not hard to do. Check out the thread on grey tanks.