There is a way to remove the tank from RV (by a hacksaw) and then heat up clean water on stove to defrost it. But character at start of the game probably doesn’t have a hacksaw and skill to use it.
Adding hot water can defrost the water in tank, but you will need a hot clean water for that.
As I remember, there was an advice in this topic Winter-proof vehicle tank, to mount car heater near the tank.