A motorhome has a lot of heat gain area i n the front are and BIG windshield.
You need to start both A/C units and put them on HIGH cool. Set the thermostat(s) for about 10 degF cooler than the room. Using a thermometer and not an infrared heat gun, take the temperature at the air inlet to the unit. Next take a temperature at the air outlet as close to the unit as possible. The leaving air should be 18 to 20 degF lower than the inlet air temperature.
Any more or less than the 18 to 20 degF spread indicates a problem. I have seen some really horrible installs and unit assemblies that let the outlet bypass air back into the inlet. Pull down the interior grill and see if the baffles are properly sealed.
How long is your motorhome? 36' to 40' should have two 15,000 BTUH units.
Ken
Amateur Radio Operator.
2023 Cougar 22MLS, toted with a 2022, F150, 3.5L EcoBoost, Crewcab, Max Tow, FORMER Full Time RVer. Travel with a standard schnauzer and a Timneh African Gray parrot