The fuel load of an A320 is 19.0T, the A321 18.9T. Additional Centre Tanks can be fitted to either, this increases the load to 20.9 or 20.8T respectively. A second ACT can be fitted to the 321 increasing the total load to 22.7T. At a fuel burn of 2.4/hr (320) and 3.0T/hr (321), this doesn't add up to a very capable tanker even with the major modification of additional cabin tanks.
Think big. From my expereince, and in my humble opinion a strategic tanker should not have any less than a total fuel load of 80T.