Prevent wasp nests under trailer?
I'm in the pest control business and if I had a product that would keep wasps from building nest on or around houses, campers, and any and all buildings I would be a millionaire!!!
The best thing you could do is fog under your RV with a flying insects spray that contains permethrin. It is a very safe insecticide. It will flush the wasps out to where you can see them and then go treat there nest directly with the same spray. It is very fast acting and there is very little chance you will get stung. Not to say a rogue wasp with a vendetta won't hunt you down and take vengeance on you!
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If you think you are to small to make a difference just try sleeping with a mosquito –- The Dally Lama