On Jan 2, 2006, about 3am, I found that exact same issue... Thankfully I had a US-Gear Unified Brake Decelarator installed on the towed for brakes so I just turned on the parking lights and got it fixed 2,000 miles later. (Somewhat rounded).
(This brake system includes a tie line from the motor home battery to the towed's)
Upon testing (I have a tester) I found the motor home was not providing The Dealer who fixed it (Warranty issue) said there was a broken wire under the coach. That's all I can tell you.. No skin off my knuckles (I normally skin them up some when I'm working under the coach).
You can get flat-4 testers at most towing shops (U-Haul hitch world) and auto parts stores and rv stores, I have, I think 3, of 'em, can not find one, One is plugged into the flat 4 (Since I do not use it any more, I now use a round six pin next to it) and one is stored in my electronics work shop.. This one is a pass-through unit, it has a flat 4 plug, a flat 4 socket, a very short pigtail and 3 LED's.. You plug it in and if the leds all light, you then plug the umbilical into it. IF Leds still light you move it to the other end of the Umbilical and plug in the car to it, If the leds do not light problem is Umbilical, if they do problem in Car, likely the diode kit or the connector.
They also make the same tester for other plug configurations,, Just more LED's.
