A couple of things come to mind and I'm not an expert in any of this so take my comments with the proverbial "grain of salt."
Our 1992, 29' Classic does not specifically have tank heaters but it does have furnace supply vents that "dump" into the belly pan near the tanks. In theory, directing some amount of furnace air into the belly pan keeps the tanks from freezing. It may be true that the tanks don't freeze but it didn't keep our water inlet from freezing or the dump valves from freezing. With that in mind, if you use the furnace supply as a method to warm your tanks you may want to consider some additional resistance strips to keep other key items from freezing.
Our 2010 Interstate has heaters on the gray and black tanks and I'm not certain about the fresh water tank. We don't have much history with cold temperatures in the Interstate as we only recently purchased this used vehicle. Regardless of the tank heating pads on the Interstate I don't see any provisions to prevent the dump valves or associated piping, macerator pump and hose reel from freezing.
I guess what all this means is there are lots of things that can freeze and may cause problems. Simply adding tank heaters may not address all the possible issues.
Lucius and Danielle
1992 29' Excella Classic / 2010 Interstate
2005 Chevrolet Suburban K2500 8.1L
2018 GMC Sierra K1500 SLT, 6.2L, Max Trailering
Got a cooped-up feeling, gotta get out of town, got those Airstream campin' blues...