This was an unacceptable solution for me as I needed to share features between modules and you cannot go up a directory.
The problem: The UI Editor looks for a gwt.xml file, then reads it, if the file it finds does not contain the path which your UI xml file resides, it decides that it is not in the package and does not look for additional files.
My Solution 1 and 2:
I solved this by creating a "aadev.gwt.xml" that included all paths needed for all modules. This one was not added to my compilation, but acted as a dummy so that the GWT designer would see it in the path. My assumption is that the paths are read alphabetically, which may be a flawed assumption.
The other solution (I tried first actually) was to add that path to the module that did work (As one always works). Assuming the gwt.xml files are close enough in configuration this should work, however if they are vastly different it may not function as desired, in which I would copy a dummy to the package path, and tweak its paths to make it work (Assuming fix 1 does not solve it for you).