[jimcol]

Having a 2006 3000RK I can tell you one is not adequate if it gets above 90 and high humidity. I installed a second unit like you are thinking. I put in a 13.5, manual operation, and a straight dump into the bedroom. It works fine and keeps the unit comfortable even when temps approach the century mark. If I had to do it again I would install a 15k ducted unit. That way if the main ever goes out I have some forced air into the rest of the 5'er. The ceiling diffuser for ducted is the only thing that is different and at least on mine, access to the ducting was already in the ceiling.