First thing I would check is the coils to be sure they are clean. If not clean them with coil cleaner and get a good air flow.
It is unlikely you have a low charge. Low charge would result in ice on the coils and the air flow would stop.
If you can remove the cover and while the AC has been running for at least 5 min. feel the suction line, the large one. It should be cold, ideally it should have water on it if the humidity is high. If it is dry and warm the compressor may have a valve leak pushing warm gas back into the suction line. This would cause poor cooling.
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