Depending on where your tanks are now you should be able to add bigger ones. If they are way in the back end you might want to consider adding bigger ones further forward if you can get the plumbing to work. That way you won't be adding a lot more weight too far back - which might unbalance the AS too much. The other thing to keep in mind is that you will either have to let the new tanks hang down below the belly pan to allow them to be bigger or mayby modify the frame cross members to allow them to be wider to get the increased capacity.
I suggest that you check out the following thread for some discussion about tanks and some sources where you can get them:
"Combined Gray and Black Water?