My camper doesn't have thermal pane windows so I made a storm window for the windows I want to see out. Just bought a piece of acrylic then cut/sanded it to fit inside the window frame. But a piece of foam seal around the edge then tape it in place when I want to use it. The rear one by the back door hasn't been removed in 3 years. The large dinette window gets removed in the spring when I want to open the window, it is removed and stored behind the dinette cushion. While it isn't exactly dual pane, it does virtually stops the condensation, definitely stops the cold draft, and rarely fogs up.
For the ones I don't care about seeing through or want light in, I just cut/sand a piece of 1/2" foam board. Even better than the acrylic.
When boondocking down to 0* F, these mods sure makes it a lot more enjoyable.