On of our favourite mods to our last travel trailer was the replacement of the analog thermostat with a digital model. Last time I used the Hunter 42999B; that model is no longer available, so this time it'll be a close copy - the Supco 45034.
My first step was research, during which I found this excellent video by Ray of Love Your RV:
My approach differed slightly and is documented below.
I like the idea of wiring the air conditioner's high/low fan switch externally. But since we rarely change the fan speed, I am going to wire up the high speed directly to the thermostat. Here's a diagram.
It matches the 4-wire setup shown on the Supco's back plate.
First step of the installation was to disconnect the battery and unplug from shore power. Due to the overcast it was a little dark in the trailer. I worked by headlight.
Here's the Coleman Mach analog thermostat.
Removing the two screws that attach the thermostat to the wall.
Here's the thermostat off the wall and a view of the wire access. There was no danger of them falling back into the wall, so no need to tape them in place.
Wire clippers at the ready. It's go time!
Wires clipped and thermostat set aside.
We won't be using blue (12 VDC negative) or green (AC fan low speed) so I've insulated them with electrical tape and tucked them back into the wall.
The remaining wires are stripped and ready. In order of connection from right to left they are red (12 VDC positive), yellow (AC compressor signal), white (furnace signal) and grey (AC fan high speed).
Here's the back plate of the Supco digital thermostat installed. I was able to use the screws and screw holes from the previous installation.
All wires are connected and tucked in place.
The front cover is on and the batteries are in. The temperature is set to Celsius.
All the power is back on and I've tested the thermostat for AC compressor, AC fan and furnace operation. It's roughly the size and shape of the analog unit and fits well in the same space.