Its been a quite a few 'mango seasons' since I have had to go into fuel tanks, however I recall they were only prime coat treated (welds were not coated) and in some cases although the ships were quite old, they were all in good condition.
FW (Pot Water) tanks were coated with bituminous black paint (similar / same as hull blacking?), I am guessing this is what you mean by bitumen, rather than tar / asphalt. If so I suspect it would be very difficult to remove (without abrasive grit blasting it off).
I would check with a paint company representative to see if it is necessary to remove (assuming it is bituminous paint) or not. Hopefully you wont have to remove it as it would be a nasty, dirty and possibly very hazardous job to remove the coating and safely dispose of the waste.