having a 500 ltr tank in a connect is stupid!,
500 ltrs weights in at 500 kgs approx depending on temperature. dependeing on frame type its normally another 40-90kgs, add a di vessel,pump battery, hose's and fitting, hose reels, waterfed poles, vehicle fuel and driver.
this will push the weight well over the required payload and should you get stopped for any reason you could get weight tested - nice fine and penalty points as its dangerous driving.
350-400 max