I use three Fantastic fans in my 30' motorhome. It keeps it well ventilated.
I get the idea of a turbine type vent fan, they will move a lot of air, but should not be up on the roof at over about 10 MPh, there willbe a lot of side pressure on it's mounts.
If you really wanted to, mount a 18" diameter turbine on a box, about 24" square and 10" tall, 2X10 sides will work great, with plywood top that you mount the turbine to, this will also add weight to keep it in place. This can sit over any one of the 14" square roofvents, and move a lot of air while parked. Make sure you put a note on the gear shift, to remove the turbine before moving the RV.
That turbine can remove all the air from your RV several times per hour, even with a 5 MPH breese.
Fred.
