You might have watched a bunch of videos on youtube on how to add a thermostat to your diesel air heater using some timer relay delay switch, it’s not a good idea. Why? because it will not work properly. The problem is the heater start up time took 5 minutes and shut off time also took 5 minutes and together you’re looking at 10 minutes of time, and if you add a thermostat to turn it off and on, apparently as you can see this will not work. Let say you set it at 70 degree to turn on and when the temperature hit 69 degree the thermostat trigger the diesel heater to turn on and run until it maybe hit 70-71 degree then the thermostat trigger the diesel heater to turn off, turning off the heater takes 5 minutes, now let say the air temperature drop to 69 degree less than 5 minutes the thermostat trigger another on/off switch while the turn off event still happening, what do you think will happen? It might be safer and better to have a on/off timer instead of thermostat.
With a timer on/off you can schedule the heater to turn on every so often like 1 hour or so at your last heat setting. Unless your thermostat have setting like turn on at 69 degree F and turn off at 71 degree, at least a 2-3 degree in temperature diff. this way allow your diesel heater to cycle through the turn off and turn on process because this process does takes more than 5 minutes.