My approach is good vodka in the occupant and cheap water in the tank.
Yea the FWC is kinda light in preparation for core winter traveling in really cold climes. What I do, or know others to do in no order of priority:
Don't use the on board water and bring jugs. My experience is that while driving the motion/rocking is enough agitation of the water that it won't freeze either tank or jug till stopped awhile. I"ll qualify that with my Grandby is older and my system has the manual/electric H2O system. That's noteable because the water drains out of the faucet/ upper line back to the tank or overall water level when off.
Run the furnance soon after stopping after stopping. Guess you could run it while driving but I haven't tried? I've heard of people using a Little Buddy portable while in motion but have not personally seen it in action.
Put a boot between cab and camper and run with the split windows open allowing the cab heat to run into the camper while driving.
Revlectix(sp) or otherwise insulate the tank and foam insulate wrap the tubing where you can. A lot of the FWC cult reflectix insulate whatever they can, like the back side of the cabinets. around the propane box and so on as a general practice to keep them warmer in the winter.
Artic Pack or otherwise insulate the soft side when stopped helps.
My camper for some reason holds a little H2O in the drain after capping. I put a little, and it takes just a little, RV antifreeze in the drain after use.
I have seen a gent who used a 12 volt heat cube in his cabinet projecting at his tank/propane box. I'll qualify that like above, his FWC was older like mine and everything was largely in one place. I have no idea where he found the cube I've never seen another one like it.
The issue for me has not been the stopping and camping but the driving when it's really cold. A lot of regular winter users get a Wave, Little Buddy, or similar heater for winter use. The reason being they are radiant(sp?) not forced air, thus they use no battery.
If you check/hunt through the Turtle expedition website:
http://turtleexpedition.com/ they took an FWC Keystone across Russia in the winter. They probably have some info and tips on winter prep/use.
Hope this info helps and good luck.