AC problem

Almost all ducted a/c systems are not sealed well at all. First, remove the cover over the inside a/c unit and note that there was a feeble attempt to seal between inlet side and exhaust side. Buy some A/C vent tape and fix the problem. Also note where the outlet duct enters the ductwork, this may also need some sealing. Pull off all overhead vents and note that the sealing with A/C tape is similarly poorly done. Some folks put a barrier (styrofoam is the material of choice) at the ends of the run if the duct just dumps in to the void beyond. Cleaning up the sealing issue greatly enhances the effectiveness of the A/C system.

If I understand correctly, when the AC fan speed is set to "Auto" it will change between high and low on its own. Is that right?

If so, then that's the basic design of the system. If you find it switching frequently (comtinious changing every 30 seconds to a minute), then it's highly possible you have a duct leak at the unit. My brand new Grand Design had this problem from day one. Over on the GD owner's forum, this problem and solution have been discussed at length. If you remove the shroud (really easy), you might find that the duct is not well sealed where it interacts with the return air duct. This causes the thermostat to get erroneous readings and short cycle. The solution is to get duct tape and seal up every single nook and cranny in there that would allow this to happen. It's pretty self-explanatory when you're looking at it. Put the shroud back on and you're good to go.

Of course..."IF" this is the problem you're experiencing...

If it's a dometic unit and thermostat, it's a standard issue. Mine has done that since new and I was told it's designed to operate like that in auto mode. I got tired of it and run my fan in Hi, or low mode instead of auto. The other odd design flaw is that if you run your fan on high or low and then need to run your furnace the AC fan will will run when your set your furnace to on. The fix for that is to set your fan mode on auto when using the furnace.

You don't say what year model, but most newer Dometic are designed to change on their own when in auto mode. Mine works great, starts on high, when set temp is 6 or so degrees lower than actual temp, and stops, changes to low, when getting close to set temp.

If yours is continuously changing from high/low, when set temp/compared to actual does not warrant, then you have an issue. If this is the case, I'd just set on low or high, as needed. Automation is great when it works!

Jerry