If you are starting up the air conditioner after the inside of the camper is already hot, it may not cool very much as the heat inside is just too much. Here are a couple tips you can try to see if you can get better service out of your air conditioner before changing or doing anything.
1) Turn on the air conditioner in the morning when the inside of the camper is still cool and it will keep it cool all day. If you wait till the camper is already hot, it may never cool it. (same as with your house, start it in the morning and it will keep up. Start it after it's already hot, it too late.)
2) If you have been traveling and the inside of the camper is now hot, and you are ready to stop for the night, keep all your slides pulled in. Turn on your air conditioner, and cool the camper. A smaller space will cool faster and even if it's hot inside, the chances of cooling are greater. Once the camper is cool, then open one slide at a time time until the temperature catches up again. This may take as long as an hour or longer, depending upon how hot it is, but it will usually work.
3). Do put a thermometer at the air output points and see if the air is actually cool blowing out.
4). If your camper camper has ducted air, chances are the air conditioner also has the dump vents on the main unit. Open them all and let the air dump directly into from the air conditioner and skip the air ducts running in the ceiling. This dumps the air right into the coach without passing through hot air ducts first, which have to cool first before any cool reaches the vents. Also, if you do the AC dump with slides IN, it will cool even much faster if the coach is already hot.
If these things don't work, then you really need to find out what the actual temperature the AC is spitting out air. If it's not spitting out cooler air, there's something wrong with the air conditioner. If under warranty, talk to your dealer. If it's cooling, just not cooling the coach, then the problem is simply the space trying to be cooled and the air conditioner is just not hefty enough to over come.
Good luck.
