Screen Removal - How To?

Make sure there aren’t plastic inserts in the tracks if you think your screens can’t be removed. It’s non-obvious, but they are almost certainly there.

I see yours windows are the older style most of that age have, all my old campers were similar, there usually spring loaded, just try and coax the top and bottom would be my guess in your model, that's what I used to do till I figured out which side held the spring. My pass through is the only one that's removeable now on the newer N.L.,the others retract into their frame's.

Spring type clip on mine. slightly press down or up to compress and out they come

Often times the screens will just slide out of the way.

Picture?
Almost each manufacturer has different system, so there is no 1 shoe fits all.

Mine has a little plastic piece that slides out. Then, the screen will come out.