Great tips above, but with the overnight temps in the 20s and daytime temps in the 40s only thing I would do is remove water hose before nightfall and run off the freshwater tank. You stated you have a heated underbelly so run furnace as you would normally do. Keep an eye on the weather just in case a sudden winter storm decides to swing your direction, otherwise enjoy the trip.
We often camp in this type of weather and have a fully exposed underbelly, most times without any hookups and have never had issues with freezing pipes or tanks with overnight temps in 20s and daytime temps above freezing. With daytime temps in the 40s it would be difficult for your plumbing to experience a hard freeze with just a few hours of temps in the 20s overnight.