How do I use SplitLayoutPanel with UIBinder? I've tried so many different things but none of them work properly.
You'll find that SplitLayoutPanel works just like DockLayoutPanel. (SplitLayoutPanel is a subclass of DockLayoutPanel.) You may have already read about this in the Developer's Guide section on Layout Panels.
Often, the UiBinder documentation for a class is actually given in the docs for its superclass. Take a look at the documentation for DockLayoutPanel and see if it works for you.