Living in Missouri where temps can go down below zero F on occasion, I drain everything and then use my air compressor to blow all the water out of the lines. I connect the air compressor to the same fitting the fresh water hose connects and then go around to each faucet (including the toilet) and let the air blow out till all the water is gone. I bypass the water heater and drain it - I use a siphon hose to get another gallon of water out of the bottom of the water heater after it stops coming out on it's own.
After the tanks are drained, I protect the traps with some used anti-freeze from my car by dumping a couple of cups in each sink drain and shower. Some goes down the toilet for good measure.
It's cheap and quick to do. When we head out just after Christmas, all we need to do is reach a campground where it's not freezing and fill the lines and water heater (after switching the bypass) again and we're good to go (to warmer places for the winter).
Trying to protect your rig from freezing with supplemental heaters more than likely won't do a good enough job and you'll find where it didn't work the next time you try to use it.
