25F shouldn't be an issue as long as you have the furnace on and it's just dipping to 25F at night. There will typically be enough residual heat off the unit to keep the water pipes unfrozen.
It's when you get down into the teens and contionus 24/7 freezing that you need to take precautions. In this case, do use the propane furnace. The duct work usually goes thru the underbelly where the water tank is and a little heat will bleed off to keep the tank above freezing.
Single digits and colder is when you really need to take steps.