Thermostat Dutchman Denali 5th wheel

So, I bought this 2016 335RLK used and have had nothing but problems with it. 

We just embarked on our very first extended (month long) trip.  On previous trips we had used the A/C with no issues.  Yesterday morning it was cold so we went to use the heater.  I set it on heat and both the heater and the A/C came on.  So, in order to get heat and no A/C I turned off the circuit breaker for the A/C.

The thermostat is a Dometic. It is square. It has the number MK1517 on the inside circuit board. I'm not sure if the thermostat is the issue here or if its more complex than that.

Maybe your air conditioner is a heat pump also and both your furnace and heat pump were running to warm up quickly. My heat pump and furnace will both run at the same time if the differential between actual temperature and set temperature is greater than 4 degrees. It happens a lot when we first turn on the heat. Then once the area is heated, the heat pump maintains the temperature.

Nick Pasich said:

So, I bought this 2016 335RLK used and have had nothing but problems with it. 

We just embarked on our very first extended (month long) trip.  On previous trips we had used the A/C with no issues.  Yesterday morning it was cold so we went to use the heater.  I set it on heat and both the heater and the A/C came on.  So, in order to get heat and no A/C I turned off the circuit breaker for the A/C.

The thermostat is a Dometic. It is square. It has the number MK1517 on the inside circuit board. I'm not sure if the thermostat is the issue here or if its more complex than that.

Click to expand...

Does it look like this:

https://www.amazon.com/Dometic-3106995-032-Analog-Thermostat-Furnace/dp/B00A2TDJ4I/ref=olp_product_details?_encoding=UTF8&me=

Odds are ChasA has hit the nail on the head.

First of all,  the thermostat that was shown by Rene T was not the one I have.   

Now to address the other two responses.  My air conditioner is a standard roof air conditioner that I have had in other RV's.  The furnace is located in a cabinet towards the front of the fifth wheel, more or less under the bathroom. 

If, what is suggested, is that the unit is a heat pump, then turning off the A/C circuit breaker wouldn't allow the furnace to work on its own.  The furnace has a standard exhaust vent to the outside of the rig.  I'm quite certain that this is not a heat pump. 

I will take and post a picture of the thermostat.

After googling RV heat pumps and seeing what they look like, it got me to thinking about the ducting in this rig.  All of the A/C ducts are in the ceiling and all of the heater ducting is in the floor. 

Many RV heat units are as you describe, including mine. They require LP and 12V power.  Some A/C units - that cool - also have a heat pump function that can also heat the RV.  I do not have this option, but it may be hard to tell by just a quick look if yours is also a heat pump.

Nick,
I suspect you don't understand  what heat pump is. A heat pump is what use to be called a "reverse cycle" air conditioner.  Basically, it runs backward and heats instead of cools. It uses the same ductwork, but he air is a bit warmer than ambient. The air from a heat pump will come through the very same ceiling vents as the air conditioner.
Just a suggestion: turn the breaker back on and let both run and  see what happens and come back and let us know.

I ran the test with both on.  The air coming out of the roof vents was 74 degrees. while I admit this probably isnt A/C temp air it isnt heated air at all.  With the furnace running i was getting 104 degree air out of the floor vents.  Even with the large vent on the A/C unit closed the noise from the roof unit would make television watching difficult.  I also checked the model number of the roof unit and it is Dometic B59516-711JO.  Googling that number shows the floowing unit

Dometic 15000 BTU Duo Therm BRISK 2 RV Air Conditioner White

From the company that invented RV refrigerator, the Dometic Duo Therm Brisk 2 Air line is powerful enough to cool even the largest coaches.  Its reliability, economical price, efficient performance and long life make it the first choice of more than a million RVers today.
Duo Therm Brisk 2 Air low amp power models draw means you can run other appliances and still have power to spare. Quiet 3-speed blowers deliver the volume of air you desire. That's the right amount of cooling with no noise intrusion.

The contoured shroud reduces draft and helps keep the unit operating at peak efficiency.

Technical Data:

Type    Roof
Capacity range of this series (BTUs)        15,000
Power (V/Hz)    115 / 60 / 1 Ph
Fits Standard 14 x 14 Roof Opening On All Campers Since 1960
Net weight. (lbs)            74
Height 12.7 Inches
Fan speed (kW)  3.5Power
Features:

Nominal Capacity, BTU/hr:  15000
Compressor-rated load amps:  13.2
Fan motor-rated load amps:  2.7
Blower speeds:  3
SCFM, high speed max/min:  300/350
Installed Weight, pounds**:  74
Minimum Generator Size:
1 Dometic Air Conditioner            3.5k Watts
2 Dometic Air Conditioners          5.0k Watts

OK, the model number shows the unit is not a heat pump.  And your 74 degree temperature indicates that only the a/c fan is running, not the compressor. 

The thermostat simply sends a signal (+12v on a wire) to either the furnace or the a/c to indicate heating (or cooling) desired.  Some thermostats also have a "fan only" capability, so can send a "fan on" signal.  At this point it sounds like you have either a thermostat functional issue or a wiring problem.  We need to know more about the thermostat to make some preliminary guesses.  Dometic doesn't label their thermostats with model names or numbers, so how about taking a photo of it and posting here. We can identify it that way. The alternative is for you to Google Dometic thermostats and find one that looks like yours, posting a link to it here.