Decorating the inside of the truck comes down to ease, warmth and visual appeal. Multiple uses would count high, also. The internet blog world is full of helpful advice.
Rugs help prevent heat loss through the floor. Also, they are generally warmer to the touch than the bare floor, and so offer a warmer surface to stand on. I put a short 28” throw rug down in the galley and another 28” rug where I step out of bed (and store them away in warm weather, for the same reason). Once the rig warms up in the morning, I pick the two rugs up and put them away for the day. (On top of cedar tongue and groove? See below.)
– cedar tongue and groove (closet lining) down the center of the bed liner. Smoothed out the floor and kept my bed dry when my rig popped a leak one night.
Now for windows:
The reflectix in the windows is a huge advertisement to everyone that you are camping in the van, or have things worth stealing in the van. I would suggest having something between the reflectix and the glass (cheap fabric?) so the metallic look is disguised.
Felt has been suggested. How about micro fleece?