Fision=fusion?
No, but fision=typo in my post
If I recall it a'right, it's actually multiplied by a factor of 9. You need an oxygen atom for each hydrogen pair, and the single oxygen atom's 16 times the mass of a hydrogen atom, so you need to add 8 kilo's of oxygen for each kilo of hydrogen aboard. I can design a basic g-carrier on a TL15 fuel cell that runs 2 weeks on 3 tons of hydrogen, but I need to bring aboard 24 tons of O2 if it runs without benefit of atmospheric oxygen for that time. On the other hand, about a ton of O2 will give me about 14 hours of extra-atmospheric flight, so I can do it with a smallish O2 tank as long as I transition from ship to planet or vice versa in that time and top it off when aboard ship or in atmosphere.
You're right in the weight relation among oxigen and hydrogen, but franky I don't know about the volume needs for oxigen (though you're right weight may be the main factor in a grav vehicle). But see that the change from fusion plant to fuel cells lowers the weight in over 6.5 tons, that can be used for the oxygen...
In any case, I guess you'll have to call on base with your G-carrier before those 3 weeks (at least for the crew, as it has no extended opperations), so reducing autonomy would not be a great problem. And the water produced by the fuel cells can be hydrolized and reused if you have the power needed at base (and I guess you'll have, even if the "base" is just a ship).
(Those high-tech fuel cells are rather magical, methinks.)
I don't know a dime about fuel cells, but I'm afraid what should be reviewed is the fusion fuel efficiency in MT (or maybe both, not sure)...