Hi waylonsdad,
You might want to take a look at our owner-written
Water Systems Winter Usage Guide for tips on how to keep the water systems running in extreme weather.
The enclosed underbelly will help some, but your furnace just won't pump enough warm air into the underbelly to keep it above freezing when ambient temps drop below zero.
Without tank heating pads, the biggest exposure you will have is tank contents freezing, causing damage. Frozen water lines, gate valves, and tanks will cause you significant inconvenience.
We've spent 5 winters in the Colorado mountains down to -30 (F). I have tank heaters, heat tape on all my water lines, and a foam-board mini-skirt with a heater under the critical plumbing areas to protect the gate valves. I like to wake up with running water and I don't like to find frozen gate valves when it's time to dump the tanks.
You'll need to arrange for an external propane tank rather than use your on-board tanks. A 125 gallon tank will do the job. A couple of good ceramic disc heaters will be handy inside to even out the interior heating. But you still need to run the furnace a lot to heat the underbelly.