An RV a/c is in two parts, one that goes outside (on top of the roof) and another that is inside, bolted to the roof unit from the underside (ceiling). The ceiling unit is where the ducts connect in a ducted unit, and where the controls are mounted on a non-ducted unit. Ducted units are always controlled by a wall mounted thermostat but non-ducted units have integrated controls that include its own thermostat.
In theory only the ceiling unit differs between ducted and non-ducted, but you need to make sure the two halves will fit together. When you replace an existing a/c, you can buy a new roof unit or a new ceiling unit or both. If you buy both units, you still have to deal with wall thermostat control vs integrated controls - you can't just plug in the thermostat wire into the ceiling unit if there isn't anything there to plug to.
To make matters worse, the wall thermostat interface on some older ducted units is incompatible with newer models.