20s is cold and creates risk of freezing for anything not within the heated compartments. What is in or out depends on your own model. However the outside sink is certainly at risk and the lines running to it. The city water port, black flush port and external shower or sprayer are all at risk of freezing if they have water in them. Low water drain points and the fresh water drain line are outside the box so also at risk. Perhaps that's enough to indicate water in the lines will put you at risk of freezing.
The Arctic packs are intended to get you over the hump a bit and will heat the gray and black tanks to help prevent freezing. I can't say how cold they will protect you to. However, they will not protect any other components of the water system but those tanks.
Don't forget your water heater. If it's on it will stay hot of course but again that doesn't speak to what's in the h/w line remote from the heater. If it's going to be off you can (should?) drain it.
The heat pump is more of a chill chaser, yes it does put out heat to a point but of course it doesn't circulate throughout the unit and just blows into the living room. The heat pump coils will freeze up below around 42 and will start automatic defrost cycles (if yours is set up like mine) during which it reverses and heats the coils while blowing the cold air into the MH - not pleasant. The heat pump will shut off below a certain point (30 I think - can't find the spec sheet at the moment!). The furnace is what is intended for serious heating needs.
If you have shore power you can use an electric space heater. Just watch the amp load on your system with everything else you may use. Watts = Volts x Amps. So if you have a 2400 watt heater at 120V, that's 20 amps. Other items likely to draw a lot of amps are anything with a heating element such as hair dryer, coffee pot, convection oven. Also microwave oven draws a lot of power. You can turn the heater off when using those if needed. If you use a space heater, you can use the heat/ac fan on low to circulate air throughout the unit. The blowing air may or may not be comfortable to all (this is basically blowing "cold" air to circulate the air when you aren't using the furnace so it's not blowing "hot" air). With space heaters be very vigilant about fire prevention (towels, rugs, curtains, children, pets).
Your propane furnace will work very well if you are willing to use it and it will circulate through the air ducts and heat the whole unit nicely. It runs on "propane only" so to speak but requires electricity to run the fan as well as of course a small bit for the thermostat and igniter.
Cold water options - bring it in jugs from the store or home, or in one or more large, collapsible 5-gal. water container.
Keep a bucket or gallon jug in the bathroom for flushing use.
Anything you keep inside the heated compartment will be fine.
If you need hot water, use the stove or microwave.
I hope some of this helps.