It is unlikly that the transformer is the problem, but the test suggested by Electron makes sense, as it completly eliminates that possibility of transmission line problems. When soaked the trees will cause the symptoms that you are having.
As for the van, you can rule that out as a potential problem. I have a van with antenna mounted as low as 24" and 25 feet of extension. I use it daily for signal testing and have never had a near by reflection problem at any heigth. See the van on my site
www.tripleplayantenna.com installation and site survey page.