You are correct on the IR thing, it indeed needs a "line of sight" from keyboard/mouse to reciever to work. Therefore you will need to mount the reciever so it can "see" outside the cab.
RF on the other hand, does not need to be mounted on the outside, it will send and recieve right through the sides of the cab. Just think of it like your cordless phone, the reciever may be downstairs in the living room, but you've gone upstairs downstairs and even outside with it haven't you? Same principle but with less power, so keep in mind that the wood or mdf of your cab will decrease the range of the RF, but how far would you realy want to be from the cab while using it anyway...