RV Holding Tanks | Replacement Rv Holding Tanks
They have about any size and shape you can imagine and will put the fittings wherever you want them. Take your wallet.
There is generally a small selection at the rv surplus joints around elkhart, but that is potluck and build the truck around them, particularly where the outlets/fittings end up.
One thought, there should be a ton of headroom in that dry box, you may be able to build the bathroom on a raised platform with the black and gray tanks directly underneath the bathroom on the floor of the box. that way you could use flat rectangular tanks and not have to try to figure where exactly to try to shoehorn them in underneath. And many rv's have the freshwater tank underneath the bed, same reasoning.
Don't forget to think about weight distribution when positioning your tanks, 100 gallons of water = 800# and can affect your balance and axle weights.