Check your manual for the description of what each circuit breaker powers.
If there is a circuit beaker in the panel that supplies the jack it should be a self resetting. Look at the panel and test the output of breaker in question for 12 volts. If you find that has no power on the output side take that wire and put it on any of the other breakers just long enough to test the jack. Assuming the inline fuse is in place. If that test causes the jack to work replace the faulty breaker an rewire to it.
If all of the breakers tested good and you have no power at the inline fuse the wire is open somewhere between the panel and the fuse holder. There may be a removable panel under the front drivers side of the trailer. Open the panel and make sure all the wire nuts are on and tight, especially noting the colored wire from the jack.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles