Use a temp probe or infra red thermometer and stick it into the vents when the system has run for about 10 minutes. Run the dash AC on high fan and temp and in the recirculate mode.
The temp from the vents should be cooler than the ambient temp in the coach. Actually, it should be about 20 degrees or more cooler at the vent if the dash AC is working correctly. I have had mine at 47 degrees on a 80 degree day.
My DP holds 5 lbs of 134a. One can only know if you have enough 134a if you put a gauge on the high pressure side and take a PSI reading and compare to the temp graphs available online.
When the air is running too warm at the vents for good AC, check the center of your AC compressor pulley driven by the serpentine belt. The center piece is the AC clutch. It will be magnetically drawn to the pulley when you turn on the AC switch at the dash and there is enough pressure in the system.
If the clutch is not spinning it usually either does not have power or there is too little 134a in the system to create adequate pressure. If not enough pressure, the low pressure switch will likely turn off the power to the clutch.
If the clutch is not spinning with the AC switch on, disconnect the clutch wire pigtail, about 12 inches behind the compressor, from the power wire and take a voltage reading at the power wire. It should be about 10 dcv or higher with the engine running and the AC switch on.
If no power, check the fuse for that wire.
If the fuse is good, replace the relay that controls that wire and probably introduces the condensor fan system in the switching process. It will cost about $10. Or switch the relay from an working system into the AC system and see if power is at the rear of the coach.
If the fuse is good, the relay is good, and no power at the Clutch power wire, then you may have an AC switch problem.
If there is power at the clutch power wire, but the clutch does not spin, take a resistance reading at the clutch wire. If less than 3.4 ohms, you have a clutch coil magnet problem. The coil at minimum would need to be replaced by removing the clutch and pulleys. This could also occur if the clutch is continually blowing the fuse when the AC switch is activated.
Touch a jumper wire from a 12v source to the clutch wire if you get a good resistance reading. If the clutch engages then the clutch is likely ok and pressure may be your problem. (I just had this experience where the clutch reacted and the ohms readings were marginal but it kept blowing the fuse on the AC power wire. So I replaced the 17 year old compressor. That fixed the problem.)
The AC techs should have checked all the above already but they shy away from big motor home electrical issues as I have learned. In fact, it is my opinion that some certified techs use shortcuts to recommended procedures to spend less time on each unit.
1995 38' CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs
Gillig Bus Chassis