Note that your ducted a/c is actually two components, one section on top of the roof and another inside. You already have the inside part with the duct connections, so need to get a compatible rooftop section. The rooftop unit contains the compressor and such, so its the key part.
Is yours a heat pump or a/c? If it's 13,500 btu, its probably a heat pump.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL