OK, I have a like but different problem with my F-550. It works as it should when a trailer is hooked up to the rear receiver hitch, however, From time to time when I am pulling a trailer with the use of my BW goose neck hitch I will get the trailer disconnected message. This will usually happen when I first hook up or going down the road and either hit a rough patch of road where the trailer bounces or a big bump. The only thing I have been able to account the message to is the lack of good ground from truck and trailer due to lack of hitch to truck contact during the bounce. However during the disconnect message the trailer brakes will activate and do their job.
Could it be that you need to clean a little dirt and grease off of your trailer coupler and allow a better hitch ground to complete.
And yes I know that if both the trailer and truck wire harness are wired with the ground wire properly attached this problem shouldn't exist. But it also seems that in a high percentage of wire problems on these trucks a poor ground can be found to be at fault.
Good Luck