Hey Gary, My optional 15,000 Dometic A/C has never been able to adequately cool my 30' 5th Wheel. I have to set the thermostat much higher (75/80 degrees) than I would like the temp to be, in order to have it shut off occasionally. Otherwise it will run continuously. Also on hot days I put reflective shields in the windows and foam insulation pillows in the air vents. Awnings help. Not sure if any of the single A/C units will adequately cool a large RV. You should also monitor the voltage in the RV. Many RV parks can't maintain 110 Volts AC on hot days. You will damage your compressor if the voltage drops below 105 volts AC. Voltage drops, amp consumption goes up, which in-turn causes increased heat in your A/C, which in-turn causes premature failure of the A/C I use a AUTOFORMER Voltage Regulator to assist the RV park maintain proper voltage to my RV. You plug it into the RV park 30/50 amp outlet and then plug your RV power cord into the AUTOFORMER. It works great. It will boost your voltage back into the green zone (110/120 volts)
Good reply DL. Since Gary doesn't say where he is or how large the RV is, we can only guess about the problem. Does the RV have dual pane galss in the windows? Is it well insulated? If the "291" means that it is 29' long, it is a lot of RV for one air conditioner, at best. And if he is in an area with temperatures well above 90 degrees, he probably will never win. I have two air conditioners in my 35' motorhome and dual pane glass. In 95 degree heat the front unit probably runs about 70% of the time and the rear one around 50%. And that is with a fan blowing part of the air from the bedroom into the front to balance the load. When we were in 105 degrees, the front unit would run from 9am to 9pm each day.
There is a way that you can check the unit to see if it is working properly, quite easily. Just get a dial thermometer and tape, or some other way attach it to the grill where the air enters into the a/c unit. Give it five or ten minutes and then check the temperature and move it to the outlet of the a/c. The differential temperature should be at least 18 degrees. If you a/c lowers the temperature through it by 18 degrees or more, it is working properly. The rest of the story is, how well insulated is your RV, how large, and how air tight?