I think what you will run into on most RC cars and trucks is that for a turbine to produce anywhere near enough power to make a significant charge would weigh to much and create more drain on the car as a whole than what it can provide. So you need to be able to produce more energy than the weight and drag of the turbine would take from the car and then also have that ratio be high enough to even warrant doing it.

Now, do not get me wrong as I think it is a cool idea and you should do some research to see if it is an achieveable goal. The other thing and this I do not know how you would do but would this actually charge the battery or just assist the battery so less total energy is taken from it? See what I mean? It basically does not charge the battery but only tries to provide additional power so less would be taken from the battery thus providing longer run time.

Now this one I am going to say is not what you want to hear. I know personally I would not want to put a turbine on top of any of my vehicles just for the way it looks. So this would somehow need to be worked into the body of the vehicle so the outer body is not changed. Another reason I would not want it on top is the center of gravity would make the vehicle more unstable in corners. This again is why I say you would need to try and keep this down in the chassis so the CG is not brought up to high. Regardless the body is going to have to be modified with a set of holes to allow air in and out so in order to really get the best flow this would need to be on top of the vehicle.

So first I would calculate for the average speed of these cars, what speed would a trubine have to spin to generate enough current to warrant this project.
Second, is a trubine that would fit these cars able to produce the above current when run at that average speed?
If yes, where would this be mounted to keep vehicle handling stable?
How much weight is added with this and does this produce more power than it would draw?

Anyway, i am rambling! Give it a try and see what you come up with. Who knows, you might come up with a really cool system and start your own company.? Good luck with it!!