I haven't tried any of them, but here is a list of recommended temperature sensors and apps with alert monitoring:
https://www.google.com/url?sa=t&sour...=1559326596162
When I'm boondocking in the late fall here in upstate NY I just turn on "Current Location" in my Acuweather app and check the hourly forecast in the afternoon. With my Bullet 220RBI I really only took "extra precautions" if the forecast was for overnight lows below the mid twenties, but I don't have a fresh water hose to worry about. With the "heated" enclosed underbelly the holding tanks are good in the upper 20s as long as you are using the furnace and not supplementing with electric heaters. I do open cabinet doors and remove the access panel under the shower to help get warm air to the plumbing lines, hot water lines tend to freeze faster than cold. The outside shower is another place to keep an eye on. I seldom use it, so i take the hose and shower head off and make sure they are completely drained early in the fall.