I'd say you are pulling more current than the designer of your F-250 ever imagined. Perhaps he was myopic, but you are stuck with it. What is the amp rating on the fuse(s) that are blowing?
You no doubt have a big alternator capable of pushing out the amps and a pair of decent sized 12V can suck in a lot of amps if they are well discharged. 50+ amps would not surprise me a bit. You may have to put a charging regulator on the circuit to hold the amps down. Meanwhile, a circuit breaker in place of the fuse would save you a lot of trouble. They have them that plug right into fuse sockets, so no re-wiring. One of the automatic reset types would be real nice.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL