The most effective method is foam on the walls, roof, windows at least 1.5 inches thick on the roof, I have about 2 inches on the windows and 1/2 inch on the walls. The more foam you use the better performance you get, but there is a limit in how much interior space you want to give up.
But that in itself is not good enough, you also need to build a wall to separate the front driver compartment from the rear of the van. I built a sliding foam door. Also if possible foam the side and rear doors, maybe something removable. You need to prevent as much outside air from getting in your van. To me that is the more important part, keep that outside air from coming in your van. Some people dont want to cover up there windows but that is where alot of the cold comes from.
Just doing that I get a 10 degree difference from inside/ouside of van. Its january and I still havent used a heater, I havent even had to sleep in my sleeping bag. The winters are mild here in california, the lowest temp I seen was 42 degrees, inside van was 52 thats with no heater. Before I covered the entire rear of my van in foam, I had to use the heater and many a times slept inside 2 sleeping bags.
The heater blanket you can use it over the foam to cover any gaps. I never use the reflectix. The rtech foam I use already has a reflective side.
Foam is very effective if you use alot of it.
rtech foam that I use, it cost about 9 dollars at home depot for 4x8 foot 1/2 inch sheet. Easy to cut with a boxcutter, I installed with a hot gluegun.
