At -5 things can get iffy.
The water heater will be ok if turned on - the thermostat will keep it hot. And the rest of the interior too, since you are residing in it. You may want to leave cabinet doors open to insure good heat circulation near the outer walls behind cabinets - sometimes those areas are isolated enough to get cold.
The areas of concern are where water lines are exposed, e.g. at the fresh water fill inlet, waste tank valves, and ice maker water lines (if your fridge has an icemaker). I would add some potable antifreeze to the tanks, put some insulation around the water hook-up access area (even some old carpet or rags), and partially block off the external vents for the fridge to increase the temp around the ice maker valves & line.
The fridge actually has two issues. First, at -5 the cooling unit may stop working. It depends on a cycle of vaporization and condensation and at those extremes it may be too cold for the normal cycle to work. Second is the exposed ice maker water control solenoid/valve and 1/4" water lines, which run behind the fridge at the outside access panel. The solution to both is to mostly block off the air inlets in the access door and maybe also partially block the roof vent as well. However, be very sure to re-open those when the temperature returns to 30+.
An alternative for the ice maker is to drain the water lines and valve, but that requires a shut off at the inside source for the fridge water line. Your coach may or may not have one, and it may be difficult to find if it does exist.