[PDESORM]

My 2004 Montana had no AC pressure when I bought it used. Took it to the shop and they pressure tested it and filled it, saying everything was OK. A week later, it failed again. I bought a fill-kit and after filling, checked for leaks and found that the needle in the fill-valve was sticking! Gave it a flick and that was it.
Make sure the middle of the compressor fly-wheell (the clutch) is turning when the AC is on. There is a built-in safety that kills the clutch when there is not enough pressure to prevent the compressor from burning out.
Good luck!