Shanise,
If you mean you have opened a faucet inside the RV to drain all the water out... this will Not work. You need to open the drain plug on the hot water tank it self... to empty all the water from the tank.
Be sure you use the by pass valves on the hot water tank... before you start to fill your water lines with RV Anti freeze solution. If you fail to do this.. the anti freeze will flow into the hot water tank.
If the hot water tank is un-plugged... by taking out the anode rod... and leaving it out... there is no risk of the tank from freezing. Leave the plug/anode rode out of the tank all winter. This is a good time to inspect the anode rod... seeing how much wear it has... so you'll know if it should be replaced when you re-install it next spring.
If you have a Ice maker... best remember that will need winterized as well.
Good Luck !