Depends on the accessibility of the tank. If you can get at the bottom, it is as RV Tech says. If not, maybe a chore.
If your tanks aren't exposed, you probably don't need warmers anyway. Depends on the temperatures expected, of course, but many rigs do not have them and get along fine. It takes many hours below freezing to have much effect on a tank.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL