Just an update. I took about a dozen sample measurements at various spots. Although I don't have expensive surveying tools like a Total Station, using 6 foot levels and some trig, angles varied between 11 degrees and 15 degrees, not including the max steep parts.
I would like to fill the tires. The Kubota guy said just fill it with water. I'm in CA foothills, so I'm assuming this is doable. Using some math, I calculated it will add about 95 pounds per tire. Does that sound about right?