Before checking the trailer check your AC source at the house. Make sure you do not have a polarity reversal. If the house checks out OK than start in the trailer as noted above.
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