Replacing the Window Screen in a Popup Camper?

I have seen these pop-up campers with all sorts of windows and also different types of material so that will be something you will have to consider when trying any suggestions.

I would suggest you visit a camping equipment shop/store or a camper repair shop and ask for suggestions as they should also have the material you will need (although most of it can be found a hardware stores for less money).
things to consider;
Is this one of the extra large windows or are the holes in a smaller window? Are screens really "screen" or are they a vinyl material? It may be very difficult to replace the entire screen if it is the larger one but still possible I'm sure. Do you want to have "old" screen showing on the inside or outside of the camper? If you just cut some of the screen out will the remaining screen have sharp edges (depending on the type of original material)?
I remember seeing a camper with a long strip across the screen opening (outside) and it was glued at ends and in several places and also sewed and/or tacked in a lot of places. It covered trimmed holes and did not look too bad.
I would think that whatever you decide you will probably need a curved canvas needle for sewing, strong fishing line, good glue (Gorilla or E-6000 maybe), patience and a good helper.

I would cut out the screen, leaving two inches of the old screen. I would then cut a new screen using the cut out piece as a pattern but cutting it about three on inches larger all the way around. I would then, using a strong fishing line, whip stitch the new screen in place.

When stitched in place, tuck the new extra screen in the crevice of the window, or cover the edge around with duck tape folded double over the edge. It would look better if you do the whole screen, not a portion.