Agree with Dave - it is probably moisture and/or corrosion on the electrical parts. It sounds as though it lioghts but won't keep running, right? Disconnect the wire from the circuit board to the igniter & flame sensor (all one unit) and reconnect. Maybe clean the terminal with a pencil eraser or electrical contact cleaner. If that doesn't do it, replace the igniter/sensor. They aren't expensive.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC