Rather than towed arty, model something like the American/British Priest, take the M3 Grant/Lee (or for the game desginers sake, the M4 sherman) medium tank chassis, take the top half off throw some side armor on, leave it open topped and put a 105mm Howizter or simular gun in there.
essentially the same vulnerability as the LVT4, but with a little more range than the stubby 75mm.