If you used the original trap it may be as simple as the trap is clogged. We frequent the beach ans thus an above average amount of sand ends up in the trap. I took a simple bath tub stopper, that fits the drain, and drilled a hole through the center of it. Put the stopper in the drain and apply a garden hose to the stopper and turn on the water. That pressure will move anything that has settled in the trap.
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