if a wintering bypass kit is installed at the water pump...
you could pump antifreeze into the fresh water lines and water pump, in under 10 minutes.
might take a quart or 2 but it could be captured at the taps and used over and over...
and it would even less time to undo this.
that doesn't protect the tanks...
but leaving 'stuff' in the black tank lowers the freezing point some.
the wash water tank could be drained quickly.
does the trailer have electrical power via ac?
IF you aren't willing to leave the furnace on, how about a ceramic or oil filled space heater?
these are 1000 to 1500
watts and some have thermostat like controls...
of course they don't directly warm the tanks, but they do warm the shell and interior structure, so some heat gets to the tanks.
i use 2 of these,
davis air dryer
one placed near the water pump (under the closet floor) and one inside the galley plumbing area.
they are basically a small low temp coil inside a protective case and are designed to run 24/7....
it's just enough warmth to avoid frost and moisture related issues (mildew) but very safe in confined spaces.