If you are running LED lights on that rear trailer, you can tap right off the wiring to the taillights at the back of your 5er. LEDs won't pull enough to be a bother to the wire size.
Electric brakes however, should be at least 10 gauge all the way back to the pin box. your current 2 axle electric brakes are already maxing out the 10 gauge on the 5er.
Pin box is easy to get to through the forward cabinet and fish through to the pin box. If you just cant get it that way, you can drop down one side of that plastic pan under the bedroom.