I have the RC unit but haven't got round yet to using it. To make swapping between vehicles easy I bought a separate chassis and set of stub axles and wheels to fit to the RC unit so that I can just remove the chassis from a vehicle and slot the RC with chassis unit in. However, in some cases the RC chassis unit will need altering, for example where it needs to be lengthened for use with a longer vehicle or shortened for use with a shorter vehicle. (You get all the bits needed for this, btw.)
Wolf Knight posted about using the RC unit with the 4821 fire engine some time ago.