I like re-purposing a device for RV use. As a terrarium remote thermometer, it costs about $10. As a special digital remote RV water tank temperature sensor, it would be $50. Great idea. Of course, heating the hot water to 97 to 105 does limit your supply somewhat.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.