This part of your post would be a good discussion to have on the JSBSim list [I’ve copied the post to that list]. There has been talk of making some changes to the fuel system for some time. Actually, these changes may already have been made. Dave Culp did make some changes. That code would be in CVS. Also, I do have John’s 747 code and have meant to see if we can add some of his changes, or implement the features.
Note that within JSBSim there should not be any limitations in place for how many tanks you have or use. So, I wonder where that problem might be occurring.