Most health departments do not allow the use of electronic 'zappers' to kill flies within an area being used to prep, cook or serve food, but there are UV lures that are very common in restaurant kitchens and seating areas. Their biggest problem is they work best when there is no outside light interference to detract from the UV lure.
Here is a link to one site that can provide a lot of different products. http://store.doyourownpestcontrol.co...FQUEnQodEBbhYA
Also on their site look for a product named BONANZA. It is a fly bait that cannot be used inside the trailer, but can be sprinkled outside to combat the flies and/or in bait stations. I have even put it in solution and "painted" my trash cans with it.
Hope this helps a little.