Here's a puzzler, and I'd like your thoughts. It's automotive, but what the heck?

My truck was unable to move my fifth-wheel camper the other day because the brakes were applied without my foot being on the brake pedal.

The camper dealer identified the problem, but not the cause. The truck's factory tow package applies 12 volts to the brake line thereby applying the trailer's electrical brakes. When the key is on, the brakes get 12 volts or so.

My mechanic went over everything he knew of and didn't find the cause, and he is good.

The truck will be going into the Toyota dealer later this week.

What do you think is the cause?