I have a 2005 BT Cruiser 5250 that has this option. The type in my coach is UltraHeat - https://www.ultraheat.com/ultraheat-rv-tank-heaters
. There are pads affixed to the tanks with wires running from them. My rig has a red lighted toggle switch in the bathroom to turn off/on.
No documentation about these came with my coach - I bought it used, but I think previous owner had provided all the docs he had. Can't remember now but think I figured out what brand it was by crawling underneath and looking at those pads.