I have home assistant running here to control the central heating, some lighting etc. With that it’s fairly easy to automate or manually control the eberspacher heater in my van when it’s within wifi range. I use a little iot d1 mini board to switch the heater on by modifying the standard eberspacher heater control panel. I have it to turn the heater on when the central heating starts mornings if the temperature inside the van is below about 10c, as I start work at various times I can also manually turn it on when I get up from my phone.
As above, a shelly or similar iot device, you could use a temp sensor, or a dc sensor and when something in the heater starts up and energises a wire with dc, then tee that wire off to the sensor and it will notify your whatever collecting device monitoring the shelly output that input xyz has energised.
I have that same shelly dc device watching some other old computer-less devices that work on signal voltages built before the age of automation but still useful but handy to monitor remote. https://www.ebay.co.uk/itm/126793002887
I feed them back to homeassistant as per carbon, and then that appears on everything that can view it, though you can use the shelly app on a phone without all the home automation overhead.
Shelly wifi and ethernet modules seem pretty solid (and they are esp32 based inside), their ble (bluetooth low energy) stuff I'm less sold on but these are not ble.