Defenestrator

If you're comfortable with DIY, you can get a Raspberry Pi Zero W and hook a DS18B20 temperature sensor to it.  I haven't set up alerts yet, but I have a little script that polls the temperature sensor every minute and submits the data to Amazon Cloudwatch, which then feeds a free Grafana dashboard.  Not exactly plug'n'play (some soldering and scripting involved), but minimal power draw (<1W), about $45 total, and pretty flexible.  I'll probably swap out the temp sensor for a combined temp+humidity sensor, or maybe just add it on so one watches the battery temperatures or something.

I generally try to look at truck stops via Google Maps ahead of time.  About a third of them seem to have a large gravel overflow lot out back that almost never fills up.  Some of them also have some dedicated RV spots.