At the time of the original sale I think some tanks were optional equipment. I believe Joie de Vivre (Canadian made, 1976 #88) originally had the maximum tankage configured this way:
Fuel:
65 gallons, starboard, under the bench in the walkthrough, aluminum
65 gallons, port, next to the engine room under the cockpit locker, aluminum
90? Gallons, center, in the keel under the engine, aluminum
Water:
80 gallons (×2), port and starboard under the cabin sole in the salon, fiberglass with aluminum top
60 gallons, forward, under the v-birth, aluminum
Black water:
40 gallons, midship, under the hanging locker and fridge, fiberglass with aluminum top
It is common in the Whitbys for the center fuel tank to be corroded and be unusable/condemned. Some owners have replaced the center tank either through the engine room or through the hull.
Some Whitbys came originally with all-aluminum water tanks. This may have been more common in the Fort Myers built boats?
Brewer 12.8s and 44s can have very different tank layouts from the Whitbys.
In some Canadian Whitbys the starboard fuel tank is only about 55 gallons with more room for general storage forward of the tank.
Most of these boats have been modified/upgraded/repaired (or neglected) in different ways by previous owners. You'll need to verify for yourself the location, condition, and useability of each tank on the boats you are considering.
Best of luck in your search. We love our Whitby!
Scott