I've had similar problems with the reverse of this - emptying an oil tanker. A smidgen of oil remains in the tanker, and the "wait until empty" condition never triggers. I've resorted to setting "fluid <100" as the wait condition for fluid trains. My unloading station uses three pumps and the animation shows all three connecting to the tanker. The tanks of the tanker wagon are connected. The pumps unload into partially filled storage tanks (also filled with crude oil to about 75% capacity, so there is room for the pump to push the last oil in).