Dissatisfied as we were with the lack of precision and slow response of the stock DuoTherm analog thermostat in our Starcraft travel trailer, I decided to upgrade to a digital thermostat. The Hunter Model 42999B is a popular choice so that's what we bought - for $20 at Wal-Mart on a trip south of the border.
Thanks to some helpful advice from camping friends and online forum members and to an instructional web page I was able to knock this mod off in a couple of sessions.
Before beginning I had to know how the wiring differed between the two thermostats. Between the website mentioned above and a bit of back-and-forth with my camping buddy Bill, I developed a complete understanding. Here's a visual aid:
Session 1 - the AC high/low switch
I did this work one evening in my shop downstairs.
If you want to be able to select low and high fan speeds you need to install a switch because the Hunter thermostat is not so equipped. It starts with a sub-mini single-pole, single throw (SPST) switch like this Radio Shack no. 275-0645:
One switch wire connects to the trailer's blue wire (AC fan high) and the other wire is soldered to the base of the "G" pin on the Hunter's circuit board. I happened to have some blue 16-gauge wire so I used that:
Once the switch is wired it needs to be physically installed in the thermostat. I chose this location on one side of the bottom, partly because that's where the instructional web page placed it:
Session 2 - Installation and Testing
Once the switch was installed it took me about 45 minutes to install the thermostat in the trailer and test it.
The first step was to unplug the trailer from shore power and disconnect the battery. Once that was done I popped the cover off the DuoTherm and removed the wires one by one, identifying them as I did with labels included in the Hunter packaging. I had to make a couple for AC fan high and low:
With all the wires labeled (and the red +7.5V wire severed and insulated) the blue AC fan high wire from the trailer had to be joined to the switch wire from the thermostat. I opted for a blade-style connector:
Next I removed the DuoTherm's baseplate and attached the Hunter's, then connected the wires per the diagram. Note the clipped and insulated +7.5V red wire and the AC fan high blue wire with the blade connector:
With the wiring complete I clipped the Hunter to its baseplate, reconnected shore and battery power and installed a set of batteries in the thermostat. I ran it through a test to ensure all of the systems were working and it was done!
Firstly, don't perform a mod like this unless you are very comfortable with the work it entails. If you do go ahead with it, be sure to disconnect all power to the trailer while changing thermostats - there is a risk of fire if you don't. Please also understand that the wiring described here is specific to my application (Starcraft trailer, DuoTherm thermostat, Dometic air conditioner) and may not match yours.
Pictures of the whole project are on my Flickr site.