I figured that I would add my two cents to this thread. I'm a budget minded individual...some may even call me cheap! I have a 1994 Viking brand pop-up slide in camper in the back of my 2013 tacoma...and I got it cheap! And I love it! It is in nearly new condition and I got it for $1100. I modified it to how I wanted it and the one last thing that I wanted to change was how quickly it would cool down after the [loud] heater shut off. I guess that I hadn't even looked on here to see what everyone else was doing to insulate their canvas sides for cold weather camping. I went my own route.
My girlfriend had recently got a sewing machine and has been making quilts. Because of this I found myself in quilting/craft shops with her while she looked around. In one of these shope I came across a thermal batting that has a foil thermal barrier infused into what I think is a polyester material. It is the stuff that is used for the insulating material in hot pads and oven mitts.
Here is the link to what material I'm talking about.
We bought some old, clean, king sized bed sheets from a thrift store for 50 cents each, cut the sheets and insulating material to fit the canvas portion of the pop-up camper, and sewed a layer of the sheet on each side of the insulating material. We added a few windows, sewed velcro to the top and bottom, used sticky back adhesive velcro on hard surfaces at the top and bottom of the canvas portion of the pop-up, then put everything in place. I have to admit, I have not yet fully field tested this thermal liner. That will be done in 3 days time out elk hunting. However, there is a definate cold layer of air in the space between the canvas and the thermal liner. I can raise and lower the top with this liner in place. It looks nice and adds a warm feel to the inside of the camper. And it was pretty inexpensive...maybe even cheap! Besides time, it cost about $65 shipped for the insulating material and about $45 for the 70' of velcro I needed to have a continious strip of velcro on both the top and bottom of the insulating barrier and to seal the windows that we build into it. The bed sheets and thread were all bought from a thrift store and the total cost there was under $5. So, all together it was about $115. When I see on the FWC site the cost of their thermal liner for their smilar-to-my-camper-size Eagle model camper is $475, I think I did pretty dang good! I'll give a more thorough report in the future as I have quite a bit of winter camping planned over the next couple of months, but I suspect excellent performance!