Cab heat is for the cab. It's not heating the underbelly, or even the back of the rig.
The tanks are the least of concern, they're big and allow plenty of room for expansion of ice. The problem is the dump valves, as well as the water plumbing.
My fresh tank is under my bed as well, and even with the rv heat on, that tank stays real cold, the bed insulates it I think... Cold water from that tank goes out into the skinny water lines, into the valves and junctions hidden inside walls, and freezes up. If you're lucky it just blocks water, unlucky, it busts a fitting and then you have a flood.
2013 KZ Sportsmen Classic 200, 20 ft TT
2020 RAM 1500, 5.7 4x4, 8 speed