Use one of these......

Plug your xbox into the control outlet and the TV into one of the automatically switched outlets....
Make sure your TV is on and then unplug, then plug into the switched outlet.
When you turn on your xbox the tc when turn on....when the xbox turns off the tv will turn off.
Now if you want to keep your xbox intact, you might want to look into a simple RF mod remote.
Or, just keep the xbox accessible from the from of your cab, and turn it off and on from there.
Another option is to hard wire a momentary switch, (arcade button) to the power pcb in the xbox, minor solder mod, requires decasing though. Mount the button somewhere on the cab and done.
Your gonna have to make you mind up though, either keep it in there or make it transportable.....My suggestion is buy you another xbox (local goodwills sell em for like $20.00 now), put the same software on it, and use that as your travel rig. Leave the other in the cab, you take that one out and it gets messed up your screwed.....