The major danger is to exposed water pipes and tanks If your tanks are enclosed then
1: Dump the waste tanks, you can add a quart of Pink stuff if you wish to keep the dump valves moist (or not if it's going to be used again soon) do not add fresh water to them however.
Put any of the following in the compartment with the tanks
1: light bulbs about 100 watts per side Make sure the bulb does NOT touch plastic
2: A string of C-9 Christmas tree bulbs (These do not get hot enough to damage plastic compartments)
3: CVS now carries the "World's smallest space heater" 150 watts
In all 3 cases I'd sugest using a thermal outlet such as
This one OR another like it