AC systems in stick homes only have one source of air that is used to blow the cooling air throughout the entire house. Different size ducts are used to keep the air flow consistent.
In an RV with multiple AC units, cold air is introduced in to the duct at two or three different AC units.
Since the owner as the choice of which units are to be used to keep the space comfortable, it is not practical to change the size of the ducts, since there is not a set cold air source.
The best way to adjust the air flow for your own comfort is to adjust the output vents to increase or decrease the air flow. In my coach all of the AC vents allow me to shut them off completely or open them up fully.