Dometic AC turning on & off

Mine does the exact same thing. It is the reverse of the short cycling issue commonly reported.

In most cases the issue is when the A/C unit turns on and immediately shuts off and does this over and over. I have the opposite issue which I believe is what the OP described. The A/C runs fine until the temp inside the trailer reaches its set temperature. Then the A/C unit turns off and within 1-2 seconds it comes back on.

The Auto fan speed setting will change fan speeds depending how close your room temperature is to the thermostat setting. If it is much hotter in there than the setting the fan will run on high. If it is near the set temperature it will run on low. The defrost thing is separate. That cycle can be ID'd by noting the compressor briefly shuts off and the fan speed increases. Once defrosting is complete, compressor comes back on and fan speed reverts to previous setting.

Thanks for all the replies. Makes me feel like it's not getting ready to die. I will check the duct work and pay attention to see if the fan speed is just changing. That is if it gets warm again in Ohio

I have non ducted Dometic and when set on low fan speed under some conditions it will occasionally go through a defrost cycle by dropping the compressor and raising the fan speed. Medium or High fan speed this doesn't happen.

Perhaps your ducted AC is doing the same for various reasons. Looks like you're getting some good suggestions. You can try raising the fan speed and see if it helps.

Mine does the same thing when directed only to the ducts. The unit seems to get lost on which fan speed it wants or when on low fan the coils start to freeze up so it shift to high speed and gets confused. I'm told the unit is running as it is suppose to and not to worry. Since I have a front AC unit I run it and leave the control "less ducted" and it runs better (at least in my mind). Reading this I will have to drop the inside cover and investigate.

Could it be switching fan speeds automatically? My Dometic with single LCD thermostat does this and seems to pause a bit when shifting fan speed.

Even with what you say about strong air flow in ducted mode....

If you know it's not blowing on the thermostat (wall mounted I trust) then...

Drop your inside "unit" and see if air is short cycling back into the return when running ducted. We've had cases of the ducting coming loose.