That's a novel idea and I'm interested in the results if you try it.
A couple of things to be aware of: When you punch a hole in the duct to fish your 12 volt wires through, make sure you seal that hole really good. Maybe just a very small amount of spray foam. Any duct leakage you might cause will make a bad situation even worse. Some of the smallest muffin fans are rated at 100 CFM. Every RV A/C I've read the specs on are rated at 350 CFM on high fan. You can't pull any more air through the unit than it can move. The air flow is kept at that level so the unit has a certain amount of time to remove the heat as it crosses the evaporator coil. I'm also thinking that if you have the fan in the vent and turned off, it will be detrimental to airflow.