I'm pretty much a black and white type of buy. I know there's plenty of you who see things in grays and different colors and would like to hope for the best in a situation like this, but ask yourself this question. If you were the owner of the trailer and lets pretend you didn't make the mess and wasn't there when it was made. Would you just leave it there on your neighbors property?
See how simple it is. Make every excuse in the book for their bad behavior, the fact is that they are responsible for the damage and mess created and they have chosen to do nothing about it.
If they cleaned it up, apologized, or took responsibility of any kind, than I'd say deal with them, make friends and work it all out. Since there is no evidence to support this, then we must come to the conclusion that they are trouble, will remain trouble and things will always be trouble with them.
Even if you plant the best screen in the world to stop from seeing them, what will that do to what they do on his land? Will it stop them from dumping, tresspassing or destroying his land in the future? Even if it does, what can he expect from them on their own land? Who wants to live next do a garbage dump?
Sell, take your losses and don't look back. Life is meant to enjoy, not fight.
Eddie