You need to put a thermometer directly at the vent where the cold air blows into the camper. If that is right at the air conditioner, or a roof vent, check the temperature there.
On mine, the current temperature blowing out of the air conditioner vent is 42 degrees. yet the cabinet doors in the kitchen are 71. The floor is 74, and the wall (in the kitchen is 76). The outside temperature at this moment is 80 degrees and the sun is going down.
There's nothing wrong with the air conditioner. It's blowing cold. The problem is with the rest of the trailer. Thin insulation, gaps in the slide seals, opening and closing doors, electronics running inside the camper, and the air conditioner is simply not able to keep up with the all heat that is intruding. Windows with sun shining in allow a lot of heat to transition in. There is simply too much heat to too much space for the air conditioners to keep the inside temperature 42 degrees. Impossible.
So, get an infared thermometer so you can "shoot" the temperature on any surface. You can shoot the infared directly into the air duct and get the temperature of air actually coming from the air conditioner.
Click here. Also, these thermometers are wonderful for checking the temperature of your tires, your brakes, the water temperature at the faucet, even the temperature of the surface of the road you are driving on. I've have, like 5 of them. I keep one in the camper all the time and one in the truck all the time.
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2019 Montana High Country 375FL
2014 Chevy Silverado Duramax, 6.6L Dually