Electronic Thermostat using "cool" mode to turn on vent fan
We do most of our camping off the grid so the AC isn't an option but our x213 trailer can get hot in the sun. A few years back I replace the thermostat with an electronic one with both heat and cool modes so though I would like the "cool" function to turn on the vent fan if it got too hot while we were out. We have a solar panel so no issue with power (I figure if its sunny enough to get hot, its sunny enough for spare power to run the fan). Anyway, just connected into the thermostat "Y" cooling terminal through a 12v automotive relay. The relay is connected across the bathroom fan on/off switch so it just cycles the fan like a person turning on/off the switch. With a window cracked open it draws enough air to keep trailer no hotter than the ambient air. Works great - even leave it active when storing the trailer in summer for ventilation.
No pictures as it just looks like a regular electronic thermostat and the relay is so small there is not much to see. The wiring is shown in diagram. There is almost no wiring to do though if you have already done a thermotat replacement.