Well this subject comes up often but no one seems to have covered it much at all. I admit that I am no expert but I am winter camping right now. I know that what I have done won't work for everyone but you may get some ideas after reading what I have done.
I bought some "bronze line" water line, this is more rigid than the standard white water line. Next I bought a heat tape "kit". You purchase the stainless steel braided line by the foot. Then you buy a plug end and a "gel cap". To me the advantage was I can make whatever length I need based on the length of water pipe I have.
On the instruction sheet in the kit it tells you how much "cord" to buy based on the length of water line you have. I paid just under $2.00 per foot for my cord. Now you should check your instructions to make sure whether you can "spiral" or "straight" cover the cord. As you can see I went with spiral this year.
We have been down to just above 0 to low teens for days and I have not had a failure.
Now from here you follow the directions on the kit. What I did was tightly wrap the cord around the hose and about every 6" - 12" with duct tape. Next I took foam water pipe insulation
and cover my line and heat tape. Again I taped it about every 12" to hold it together. The I used the duct tape to wrap the foam really good. You want to make sure water will not get in and eventually rust the stainless steel cord. This is used as a ground for the cord.
Here is my finished product.
I used a wire tie to suspend it from one of my slide-out to prevent it from lying in the cold snow. Behind my water door is some additional heat tape and some cut foam pressed in flat. I was able to lock the door to keep it stable.
On the water bib (water head) the RV park that I am at has a heated rod down the middle. I also left a few feet of heat tape on the female side of my water line so I could wrap the connection a couple of times. I then created another heat tape and wrapped the water filter. Lastly I took a small tarp 6' x 8', folded it and wrapped this entire area. I used some bungee cords to keep is snug. With this setup I have made it down to -10 degrees of actual temperature. I did not record the wind chill during this time.
Now I know you can buy pre-made winter line and I am sure it works great, but once you price it out you can make many of your own and still not spend as much.
This that you could change are:
Find a place in the RV to mount a water filter. If you can't then I would look for a box, mount the filter inside it. Then mount the box to a pole to stick in the ground and optionally use PVC to make a tripod, just don't glue anything together.