Sometimes I get an influx of house flies in my 5er. I've often wondered over the years how these pests are getting in. All screens are in perfect shape. IMO, one way they can be entering is through exhaust vents for the fridge and heater.
I think another strong possibility may be the holes where the piston rods for the slide-outs enter the basement. Some years ago I believe mice were getting in this way until I plugged the holes with rags (had to remove rags when operating slides, of course). I'd hate to resort having to plug the holes with rags again but if that helps I'll do it.
One year it got so bad I hung fly paper at various places but hardly caught one fly. The only method that was effective was using the good old fashioned fly swatter.
What, if anything have you found effective in deterring flies (or any insect) from entering your RV?
"Not all who wander are lost"
2008 3500 RAM/Cummins
2007 NuWa FW, 29'