If you're looking to only replace the rooftop unit and still maintain use of the thermostat inside of your trailer then I recommend going with the Advent Air Replacement RV Air Conditioner for Dometic Setup w/ Start Capacitor - 15,000 Btu # ACDOM150 or # ACDOM150B for a black shroud. These units have the 15,000 BTU rating you're looking for and are a quick swap from your Dometic unit.
To remove your old unit you'll need to take off the cover for your air distribution box on the inside... view full answer...