I,too, would be a bit concerned about using window screen.
1/8 inch hardware cloth cuts the airflow to 70 to 80 per cent, depending on the size of the wire.. Fine mesh screen could be as much as 50 per cent...
What it really means is, you would be reducing flow over the fridge working components by a big margin. That can result in higher temps in the fridge, and possible warranty issues.
Just something to keep in mind.
I suspect that the Camco vent screens are somewhere around 1/8. Small enough to keep out mud daubers and mice, hopefully big enough to allow sufficient airflow to the fridge intake and exhaust.
Back in the day, it was recommended to remove any screens installed before asking for warranty service. I'm sure this is why screens are not standard on any trailer....
Sherry