Trailer Disconnected

I recently traded in a Silverado 1500 for a 2015 expedition with HD tow package. I never had any issue with the Silverado (had an aftermarket brake control put on when purchasing travel trailer new) pulling our travel trailer which is just over a year old. On the first day hooking up the expedition to the trailer, the trailer lights would work, but the in dash would reach trailer disconnected. I took it to a local rv service and they checked out, ended up looking in the trailer junction box and changed the 7 pin head on the trailer and finally got to register. Took the trailer on a test drive and within 2 minutes melted the charge cord and trailer wiring under the vehicle. So I had to have all the wiring and 7 pin receptacle replaced by ford delearship. They rewired the trailer but still having connectivity issues. At times it will say connected, but will either immediately ding back to disconnected, or when I pull forward slightly with disconnect. The service techs feel like they have checked the wiring l, but cannot figure out why it is disconnected. I have hooked up to another trailer and a test box they have and have a good connection on the vehicle.
Any ideas or has anyone had a similar issue?
Thank you

My gauge has always said trailer disconnected as the last item in the checklist. I've always ignored it after confirming the lights work. Perhaps it's asking whether the trailer is disconnected from shore connections, not from the vehicle? Never seen anyone else's setup, so I don't know what it's supposed to say, but it hasn't interfered with me towing. I also recall TFL saying the plug can come loose from the recepticle but this has not happened to me.