If you enclose the bottom, you might also want to add a small duct to heat the space inside the enclosed bottom.  When your furnace runs, you would be dumping a little of the hot air around the tanks and can keep them just barely warm enough that they don't freeze.  

Finally, you can consider using some adhesive closed cell foam tape around the drain lines up to the gate valves.  Though the tape won't heat the pipes, it can help hold enough residual heat from the contents of the tanks to keep the contents of the pipes just above freezing.

In another post, I described a trip we took across the country and encountered sustained 10 - 15℉ for much longer than you described in your post, as we drove across Wyoming and parked for the night in a rest area near Cheyanne.  The only thing that froze were the low point drain lines for the pressure water system in the inch or so where they stuck out from the bottom enclosure.  The bottom of our trailer has the so-called "Arctic Package" [the foil backed plastic cardboard], the heater duct, and the wrapped tank drain lines.  I left the furnace turned on and set at around 60℉ while we drove through that cold.  The pex to the drain valves thawed with no problem.

Thanks for the reply. Not really "winterizing" but just draining water lines for the night. The place I camp has power and water but no sewer. I hold water inside in a 5gal cooler for use during the night, but have no way to drain the holding tanks without leaving the camp site. There are public toilets and showers but I still am going to generate some gray and black from cooking and middle of the night toilet use.

As for temps, it is usually during the last week of the trip when things start getting cold. Last trip it would drop below freezing shortly after dark and then be 17F in the morning and not get above freezing until 10-11am.

Looking into the heating pads...

When I camped there before I was in an Aframe with no bath and managed to let the water run a little all night and drain on the ground. Not going to work with the R-Pod.

One of my regular trips is to BC in the fall when temps can fall below freezing during darkness. The water lines can be drained and what not so I'm not worried about them. What I am worried about is the gray and black water tanks freezing and ruining the valves or piping. Has any one ever tried attaching heat tape to these two tanks for over night protection?

If so, how did you attach it?