Exactly. There is no free power. If you found a suitable wind turbine it would create air drag that makes the power it creates less efficient than using the alternator. If you need better charging, get a bigger alternator and increase the size of your wiring.
Never on a moving MH. I have seen some systems for when people boondock and have solar too. Looked at this setup however was cost prohibitive... plus could not leave it hooked up while underway. I see Sail boats with them all the time to top off the batts. However they are not mving at highway speeds. I would rely on my alternator to charge/maintain batteries while underway