Roger & Brenda: From the Ford Motor company web site, the owner's manual for F350 (
http://www.motorcraftservice.com/pub.../03f23og5e.pdf ), page 180 -- it shows that there is a 20 amp fuse in position 115 and it is called the "Trailer Tow Battery Charge"; this is the line that would be active to charge the battery in your trailer while going down the road. Looking at the diagram it appears that that is the 3rd fuse, bottom row, from the left of the fuse box.
Hope this helps.