At the factory, this is done before the TT Box is built. They flip the rolling frame upside down... That is the easy way to do it.
A retrofit will be much harder. If you are gonna go to this much effort, consider going all the way. I used 1" polisocroyante (sp?) insulation from Home Depot to insulate the inside of the frame rails, and to rest right on top of the underbelly. Think of making a large insulated coolercontaining all of the tanks and plumbiing. Before sealing it up, put a hole or two into the heat duct so some heat will get into the new ice box. If you are really serious (I was) install heat tape and insulation on all plumbing.
The money to do all this wasn't bad, as I bought the heat tape on ebay.
An electrical circuit/outlets will need to be installed as well.
With this setup, 40 below should be doable with running water as long as a generator is running. An EU 2000 works well.
Although I not encountered such extremes yet, the floor stays warmer, and propane use and heater run time has be cut in about half.
The underbelly by itself will only help a little. Doing the additional mods will make a BIG difference.
It just depends how far you wanna go.