LET THERE BE LIGHT...
Side and rear windows are one of the best external style changes you can make to your van, visually turning it from a commercial vehicle into a recreational one. They also bring a bunch of other benefits, such as:
Increased visibility while driving, especially when parking or manoeuvring at low speeds
A much nicer living environment when in the van, especially on dull, rainy days
They act as a deterrent to criminals, who no longer think that there might be tools in your van
Not all campervan windows are created equal, however! The best are made from privacy glass, not clear glass that's later tinted, and they should block out 70% or more of the outgoing light to ensure that your van is reasonably private in the daylight.
Big V Recommends:
...THEN BLOCK IT OUT AGAIN!
There are a lot of window treatment options available, especially curtains. They're not all created equal, however!
Blinds should either be behind a splashback or plummet, or ideally in a purpose-made pod to fit the window aperture. Good quality thermal blackout blinds, properly enclosed, will dramatically improve the insulation of your campervan, giving you cosy nights and comfortable mornings.
The next best option is thermal blackout curtains. These have the advantage of being available for far more vans and are able to be adapted for use with almost any make or model. Good campervan curtains have poppers to secure the edges to the sides of the windows and have smooth running tracks, with the screw positions outside of the track itself.