I'm running tater's campaign mod and I've noticed that he has a very good mixture of ship classes. But tankers are the exception (as they really should be from a hostorical point of view). None-the-less, I've sunk 3 in 4 patrols. 1 each in the Marianas, East China Sea and Palaus. One medium took 5 torpedos (not counting 4 misses or duds

). 9 torpedos for ~3900 tons...
In the stock campaign you could always find a couple travelling with task forces, slowing down the capital ships. Tater cleaned up that faux pas as well (thank you tater!!!).
Historically--Dutch East Indies to Japan (routed mostly west of the Phillipines (I think)) saw a lot of tanker traffic.
Keep Hunting and I bet you'll run into plenty of them sooner or later! The US is counting on you!!!
Cheers!
Peto