The most common aftermarket cap would be a STANT 10840. Pretty much any auto parts store should have it or an equivalent. The 10840 replaces about 50 Ford part numbers. If the parts store doesn't have a gas cap listing for a 2004 Ford F53, the 2003 Mazda Tribute uses the same cap.
Dutch
2001 GBM Landau 34' Class A
F53 chassis, Triton V10, TST TPMS
Bigfoot Automatic Leveling System
2011 Toyota RAV4 4WD/Remco pump
ReadyBrute Elite tow bar/Blue Ox baseplate