In theory On the RAF Hercules tanker we could give away all of our fuel by pumping it up into the wing tanks and hence out via the HDU. We could only dispense fuel via the wing tanks so the fuselage tanks were used to keep the wing tanks topped up. Max fuel plus or minus SG etc was 63000 lbs in the wings and externals and 28000lbs total in the four fuselage tanks.A full load made the a/c rather heavy !
We originally carried boiler plate bolted to the floor forward of the tanks for C of G reasons but this was later removed and 'ballast fuel' was meant to be left in the forward tanks to compensate. Once had to use this little extra !