A second truck is your best bet. And...you need a big sprayer with a high pressure and high volume pump to reach trees. Perhaps a 30 gpm pump and 3/4 inch hose. Perhaps 400 lbs pressure. Need tree-spraying gun. You want to be able to reach 60 to 75 feet high. At 20 gpm a hundred gal tank won't last long. If you are not ready for a full tree spraying outfit. Get a lawn care skid sprayer. It can do most types of applications. Scroll down 2 inches, click, and look at the Gregson Clark line of sprayers. The backpacks will work fine, if you don't plan on tall trees. Myself I don't think you need high volume for insecticides and fungicides. Fruit and vegetable growers use low volume fungicide and insecticide applications all the time. Air craft sprays are almost all low-volume. Fungicides need to dry on the foliage if you intend to coat the leaf blades.