What is your RV?
Water is at its highest volume during freezing. (that's what's causing damage)
Once the water is frozen, the volume will decrease if the temp drops further.
The vulnerable parts are not the water tank,
but the pipes and joints.
But empty the tanks anyway.
Turn off your water pump and open the faucets. You don't want any pressure in the pipes.
Pour anti-freeze into your toilet, shower pan, sink. You don't want any trapped water to freeze on you.
Open the cabinet doors, so that warm air can circulate into the under cabins.