With the pure tanker VC10s, you'd have been about right! The ex-passenger deck housed several additional fuel tanks to raise the capacity to something useful. But the Tristar, which has just retired, and the Airbus A330, which is the subject of this current teacup tornado, carry sufficient fuel in the normal tanks to function perfectly well as tankers whilst simultaneously carrying either a few hundred troops, 20 or 30 tonnes of freight, or a couple of political egos.