I have a 04 Georgetown 370XL with the Ford Chassis and I found a locking gas cap at Auto Zone. I would think they would have most gas caps in stock depending on your Chassis. You could always go to the dealer if you have your vin number and they should be able to fix you up.
Only thing if you go to one of the auto parts stores you might have to drive your motorhome there to check it. You could always get one and return it if it doesn't fit.