I get this message "service parking brake" anytime I'm towing, Doesn't matter if hilly or not, etc. once disconnected the CEL goes away. Took it to the dealer and they said I had to bring it in "when it was doing it" to tell what causes the problem. I'm not going to bring the truck and my 30' fifth wheel to the dealer. Trailer brakes work fine. parking brake is fine. I have no idea what it could be, anyone else seen this?