I too had this problem with my Ford. I believe there is a TSB out for the 2008 Fords 7 pin connector, seems they are not sealed in the back where the wires plug in and water can get in and corrode the connections.
I pulled my plug out from under the truck and all the connections were corroded, cleaned everything up and put some dielectric grease on the connections. Then I checked all the wires that I could find, no problems there. Then I had the 7 pin connector on the camper side replaced, this fixed the problem.
So, it could be a number of different problems, so you just have to start somewhere and check everything.
Here is a link to the TSB: