With my conversion project I wanted to keep the classic looks of a class 8 rig, so my solution to the gray tank question might be somewhat unique. Our layout for the camper portion of the vehicle does not include a toilet; I don't want the odor and the upkeep. Plus, where we going camping most of the time there are very nice restroom/shower facilities. When on the road, we stop at McDonald's, etc.
However, I did need a solution to the freshwater, graywater supply and storage problem. I ended up finding a used tank which matched the fuel tanks on my rig, and had it made into a "split." This combination tank is mounted behind the driver-side fuel tank. I have about 50 gallons of fresh water supply, and 50 gallons of gray water storage from the kitchen sink on board. So far, it has worked effectively.
Speed Gray, K8SG
Grand Rapids, MI
Highway OPie (Speed Gray) Grand Rapids, MI: 2003 Peterbilt 379 Motorhome; 550 hp 6NZ Caterpillar C15, Eaton-Fuller 18 speed transmission, 3.36 rear; 63" sleeper and 16' Morgan box.(firstname.lastname@example.org). There are many Peterbilts, but this one is mine!