Another thing you can do is after you have run for a while and are shure your engine is up to full temp, Carefully feel the heater hoses to see if they are hot from water running thru them.. If they are you may want to consider installing a water shut off valve. When you turn on your ac there is a door that closes to your heater core, However some heat gets by and cuts down on your cooling output.. When I worked for a bus company in the 80s we would send motor coaches to Fla. were they would run for 4-6 months before they came home for service.. We would check the water valves before they left to help them keep cool in the hot south. Your coach may already have one, But if not they are very easy to install..