Actually, Huntingdog, our real reason for not using the furnace much is not about saving propane. It is more about electricity -- the furnace draws four amps! And it is noisy.
But you are absolutely right -- the water lines usually won't freeze in the high 20s, especially if they are all inside the trailer and if you keep your trailer warm.
The real problem is the outflow tube from the water tank, which is thin and is often not inside the trailer. It will usually freeze in the low 20s, unless you have installed a heat cable.