According to the 2007 F-53 chassis owner's manual, page 66, it's fuse 22 (20A) in the Power Distribution Box (PDB). The PDB is under the hood, against the firewall, about in the middle, above the radiator.
Same fuse powers the "cigar lighter" circuit Ford provides. Your coach may or may not use that circuit.
Link to Ford online owner's manuals is here