It is condensation caused by the humidity inside the trailer collecting on the cold door frame. There is not much you can do about it other than limit your cooking, especially steaming foods, and limit the use of hot water. The moisture also comes from your body and breath. You could try a dehumidifier, but I doubt you'll find the results worth the expense.
2013 Ford F-350, 4x4, Crew Cab, Long Box.
2001 Jayco Eagle 266 FBS.
2014 Heartland Elkridge 37 Ultimate.