2004 5th wheel
13 years old
Good possiblity the auto reset 12v breaker died, or the connections are rusted all to H****
From the batteries trace the the black Aka positive wire until you find a little silver/Gray rectangle with 2 studs and wires
Check voltage at both wires to ground,
Replace it, buy a new one at your favorite auto parts store
Most likely a 40 amp, but look for any markings that might be on there to indicate amp size, they function as thermal circuit breaker
They are exposed to the weather, and know to died,
13 yrs, a good life