Voyager does not have any additional fuel tanks; unlike the KC-46A it doesn't need any.
Voyager's under floor cargo area isn't compromised by additional centre tank 'plugs', whereas KC-46A's is. Hence it needs an upper deck cargo door whereas Voyager does not.
Once again, "need" is in the eye of the beholder.
Your eye might not see those as needs. But the USAF eye decided that a cargo door and cargo floor were priorities. The KC-10 for example (which is even larger than the A330) has a cargo door, cargo floor, and belly tanks. Why? Besides the ability to handle fully loaded military 463L pallets, the cargo door/floor facilitates medevac. Airbus not even offering them hurt our proposal. We dearly wanted Airbus to offer an A330-200F based MRTT, which had the cargo door, cargo floor, and the revised nose gear. But they did not want to move up the freighter development schedule to meet the first competition's schedule. They simply refused to offer it for the later competitions and still refuse to do so. Don't know why.
And oh yes, if Voyager has no belly tanks, then I don't know how it could possibly mass out before it volumed out. It's hard to imagine Voyager's empty weight is 20klbs higher than an A330's empty weight. I do know that the MRTT offered to USAF in the first round was very slightly lighter than the A330-200 even with the addition of all the refueling gear because it had the lightweight passenger floor, and no seats, galleys, lavs, etc. I can't remember for sure, but it may not even have had the cargo handling gear in the belly. It was highly optimized as a pure air tanker.