I was reading my new Dometic refrigerator instruction book...and it said to put the 'winter cover' on the outside vents if the air gets below 50, and when washing the trailer. Hmmm no 'winter cover' included, and Dometic just told me that the 'winter cover' is not available in the USA....AND...that
if I try to use my Dometic fridge below 50 degrees, the coolant will gel up. Huh? Does that mean I can't use my refrigerator below 50 degrees?
What happens when we're driving along with the fridge on battery, and the temps drop into the 40's? What happens when it's 65 daytime, and 42 overnight?
SURELY we aren't going to have refrigerator problems if we have it turned on and the outside air temperature drops?

I continued googling for more info...and just ran across this that is v-e-r-y interesting!
Your RVs Refrigerator is very vulnerable in cold weather. Your RVs
refrigerator has chemicals in the cooling unit that will gel up like
diesel fuel if the outside temperature gets down too far. This will stop
your refer from working properly. There needs to be a certain amount of
heat in the vented compartment behind the refer for it to operate as it
should. For that reason, it is important to park your RV so the back of
the refrigerator gets as much southern exposure as possible. While this
will not guarantee that your refer will continue to work, this will
help. If that is impossible to do or if it is extremely cold, you can
open the access door on the outside of your RV and add a bat of UNFACED
FIBERGLASS INSULATION to cover the vent holes in the lower compartment
access, thereby slowing down the circulation of air through the
compartment and keeping some of the heat in. A single bat of unfolded
and unfaced R11 should be good enough.You are not trying to completely
block off the vent... you Just want to slow down the movement of air
through the compartment on the backside of your refer. NOTE: Use only
UNFACED fiberglass insulation as it is not adversely affected by heat
sources or open flame. AS a suggested rule, place the bat in the
compartment when the outside temperature gets down to and stays below 15
degrees... just remember to pull the bat out the next spring or the
refrigerator will run too hot and will stop working as well. NEVER cover
or block the upper vent. In conjunction with the UNFACED fiberglass
bat, I've seen the use of a small shielded 60 watt bulb in the bottom of
the compartment to help further provide heat to the cooling unit in
cold weather as well. Just insure if you use the bulb, place it in a
drop light socket with shield and place it away from wires or anything
else it may come in contact with. THE MAJOR REFRIGERATOR MANUFACTURERS
FROWN ON THIS PRACTICE SO, IF YOUR REFRIGERATOR IS STILL UNDER WARRANTY,
YOU MAY WANT TO FIND AN ALTERNATIVE MEANS TO KEEP YOUR FOOD COOL. The
major manufacturers provide for no preventive maintenance in this area.
It was explained to me that an RV refrigerator goes in an RV...
Following that thought, an RV is what it is... a RECREATIONAL VEHICLE,
and the use of their product was not intended for extreme cold weather
operation. Thus so, the procedure for a frozen cooling unit is to remove
it from the RV and let it thaw in a heated garage or room for at least
72 hours or more and then retest it for operation before re-installing
it. Why wait for it to freeze in the first place? Preventive maintenance
is the key.
As a note, I've also seen owners fill the outside of the refer access
panel with the grey or white split styrofoam pipe wrap. They just cut
them to length and fill the three vents, so commonly seen on Dometic and
Norcold access panels. These syrofoam tubes fit snug and, because they
are on ther outside of the access panel, they make more room for a drop
light and are seen easily so, come spring... you will remember to remove
them.
So ,how do you know when your refrigerator is froze up? It is a pretty
good indication that the chemicals have gelled up is when:
The outside temperature has been below 15 Degrees F or less for several days
The refrigerator has stopped cooling on the inside
Touching the Chimney (Commonly a silver vertical tube on the right side
of the back of the fridge behind the access panel) is warm while at the
same time, the absorber vessel ( a big grey or black horizontal canister
sitting the middle of all of the tubes in the back of the fridge, also
behind the access panel) is cold. To prevent damage to the cooling unit,
immediately TURN OFF YOUR REFRIGERATOR AND CALL A SERVICE TECHNICIAN.
NOTE: AFTER your refrigerator is froze up, the ONLY thing an RV
Technician can do is remove the unit and place it in a warm place for 48
to 72 hours to let it thaw out. He must also test it for proper
operation before re-installing it in your RV.
(source of this info is this forum page: http://www.airforums.com/forums/f425/extreme-cold-affecting-fridge-72206.html)