You should have a separate fuse box for the seven prong trailer towing light socket - I found mine in the compartment with my inverter - different coach manufactures can and do put them in different locations. below are the pictures and description of where mine were hiding... good luck!
I found the fuse box that houses the fuses for the towing light socket! On my 2007 National Tropi-Cal LX T350, the fuse box is located on the curb side in the back baggage compartment - behind the metal screen in with the Xantrex Inverter. There are two small fuse boxes mounted on the back wall - the one with the number 2 on it housed the fuses. I found 3 blown fuses, left turn/stop, right turn/stop, and marker. After replacing them the towing light plug socket works fine. Hope this information helps somebody else!