If temperatures are expected to be below freezing, the motorhome rental company will have prepared for the conditions by winterizing your RV. This is important to protect the vehicle's plumbing system against damage.
To winterize the vehicle, the motorhome rental company will drain all of the water from the vehicle. The system is then pumped with antifreeze. Representatives from the motorhome rental company can provide you with more information about this process.
You will not be able to use the water system if you encoounter freezing conditions, so you will need to bring bottled water. If the conditions are unexpected and there is still fresh water in the vehicle, you should keep the water heater running to try and lessen the damage to the system.
Generally, renters will be required to return the vehicle winterized if this is the condition in which it was rented out. You will be responsible for the cost of any damage incurred if the vehicle is not kept properly winterized in freezing conditions.