I have a dead battery on my 1150RT I need to replace. I know it is behind the tank but what do I need to remove to get the battery out and the new one in?

You need to remove the left side body panel to access the battery. You have to first remove the engine spoiler (4screws) and then all the screws around the left side panel. There is one screw behind the mirror which just snaps off the posts. Just whack the mirror had with your hand towards the back of the bike and it will pop off, then remove the light socket and set mirror aside. Remove the seat too, as there are a couple of screws hidden under the seat. Continue to remove all the screws around the body work and then lift the body panel up and out away from the bike. You will then see the battery behind the ABS modulator. Remove the wires from the battery as you slide it out of the tray.

This whole process is kinda difficult and requires taking your time. If in doubt best to take the bike to an experienced BMW mechanic.