One can test the unit's performance with ordinary room-air thermometers, too.
Here are some instructions for doing so from a very illuminating page on the subject of RV A/C ( link to source)
The RVer can perform a temperature performance test with two thermometers. One is used for ambient room temperature going into the A/C filter and the other thermometer is to measure the cool air exiting the discharge vent closest to the A/C unit. The A/C should be set to maximum cold and should run for at least 30 minutes.
The factory specification is approximately 20°F cooler air exiting out the discharge than the ambient room temperature entering into the filter. Air conditioners only remove the heat from the air, so if your RV temperature inside is 90 degrees the discharge temperature would only be approximately 70 degrees.