bethann, Yes! I have used a floating stock tank heater to thaw out a frozen septic tank. It worked quite well. Don’t worry about the sewer pipe from the house to the septic tank. The sewage flowing through the pipe will not freeze. The flow through the septic tank is very slow, so there is a potential of the sewage freezing if the soil frost depth is very deep and the septic tank is shallow.
Another approach to “prevent” freezing is to place several bales of straw or hay over the septic tank. If the sewage in the septic tank is already frozen, then it must be thawed before the straw bales are placed. It may be prudent to place bales over the leach field also.