We got our 2007 F-350 6.0 (w/towing package) and 5th wheel RV a year ago. Right from the get-go, I would get the dashboard message: CHECK TRAILER, and most of the time, it was there very momentarily, after hitting a bump in the road.
Once in a while (not very often at all), the message would remain solid, and I'd have no trailer brakes, so I'd pull over and unplug & reconnect trailer cable on side of bed, and all would be good.
On a current trip from FL to CA, in east Texas the trailer brakes would go to 100% application with the lightest of braking (apparently whenever brake lights were activated), and the message: TRAILER FAULT would appear on dashboard. The trailer brakes even came on 100% whenever turn signals or 4-ways were on!!! (i.e. whenever light(s) for them on dash were ON).
The problem suddenly stopped (thank goodness before we went thru a thunderstorm). The next day, problem was back & then stopped again. The next two days on the road, problem did not happen at all.
While here in LA, while driving around (without RV), I began having TRAILER FAULT indication on dash (whenever applying the brakes), meaning problem was with the truck, not the trailer.
BTW: My normal trailer brake application setting is "8" (out of 10), but when I turned it down to "0", trailer brakes still applied just as hard during the fault condition.
BTW II: I check my OBD II with my smart phone, but no errors show up specifically about trailer.
I just found out there is a recall on the ignition switch whereby brake light may not work due to a problem with it. Could that be the problem???