I just installed a Roadmaster system on my 2011 Discovery 40x. I tow a 2011 Honda CRV. I used the Roadmaster adapter plates, a Sterling All Terrains tow bar, and a Road Master Invisibrake system. Great combo system. Just towed to Florida from Ohio with no problems. It took me about 4 evenings to install the Invisibrake in the CRV, but it is a great system. I mounted the compressor on my spare tire in the CRV and the operating cylinder under the drivers side rear seat. I ran the wiring up the drivers side running boards to the rear of the CRV. Used a 4 wire system and strung an additional wire for a spare if needed in the future. I wired the compressor to the 12v outlet in the rear of the CRV, using a fuse for protection. I also hooked up a hot wire from the 40X hitch to the battery of the CRV. This keeps the battery charged and I don't have to pull the fuse on the Honda when towing. Roadmaster has some good slides on installation of the systems on their Website, but they do not provide information on recommended installation areas for the invisibrake cylinder or compressor system. I can provide pictures of my installation if anyone is interested.
PS, it works great with the Jake Brake on the 40x once you figure out how to feather the fuel pedal during Jake operations.