Before you get too excited about checking the wiring harness, double check that you don't have a second fuse box.
Ford for instance has a second fuse box that contains the fuses for any trailer being towed. Some coach builders use the OEM trailer lighting package for the motorhome body itself. That would explain why your trailer lights don't work either.
Ted 'n' Laurie, plus Jackson (aka Deputy Dog, the Parson Russell Terrier 'fur kid') and, Rylie (who crossed the Rainbow Bridge June 14, 2012).