We have an 04 wrangler and use the same braking system. As I remember the instructions say to tap into the cold side of the brake light switch connector at the brake pedal. Just unplug the connector from the switch. Stick a grounded test light into the socket to determine which side is cold. Then use a Scotch Lock or similar connector to connect the LED that wire. There is no need to run a wire any further.
Here's a link to the installation instructions:
http://www.roadmasterinc.com/pdf/85-1811-08.pdf
Page 3 shows where to connect the wire for the LED. It's at the right side of the page by the diagram labeled: MOTORHOME MONITOR
You will probably need to install a set of diodes at the Jeep brake lights. If the brake lights on the Jeep come on when you step on the brakes in the motorhome (or when the exhauust brake comes on) the power will feed back through the entire circuit. You won't know if the LED is comming on because the brake lights are being activated by the BrakeMaster air cylinder, the coach brake lights, or the exhaust brake. Once the diodes are installed there will be no feed back. The wire at the cold side of the brake light switch will only be hot when the Jeep brake pedal is depressed by the air cylinder.