A good cleaner that I have used for flies is Mer, it also cleans the majority of the black streaks from fridge use and water stains. The instructions state that it is also used as a polish, but I have to admit that I prefer an alternative polish. Mer can be used safely on caravan surfaces as it doesn't wear away the thin aluminium paint layers like a T-cut type cleaner would. For cleaning a plastic front screen, you would possibly be surprised to hear that Brasso, yes the metal polish, is good. It contains petroleum distillates so it shouldn't be left on too long as it could do damage to the plastic, but rub on, polish in, allow to dry to haze, and buff off will clean the dead beasties and using a soft cloth for the buffing, it will bring back a nice shine to plastic windows that have got fine scratches or gone dull. Soft cloths are important for all the above since you don't want to scratch the final coatings on your van.