Wow, are you serious?
If you are thinking of running an under-CDL truck you easily would be over your weight limit with just the two skids and 1500 gallons of water. It sounds like you've given no thought to what each axle is rated at either. Unless you put the water tank near the rear of the box and close to the rear axle (which kind of defeats the purpose of a 24 foot box) you are carrying too much weight on the front axle. Even Paul K is close to doing that with an 18 foot box unless he's got an over CDL truck now, and Paul has years of experience and knows what he's doing. What happens when that water weight shifts to the front axle when the brakes are applied? That would be great if you shear the axle or steering linkage. Plus, is the frame designed to carry that kind of weight over that great a span? Probably not, so you'd need to reinforce it, which adds to the weight. Now, what do you do about the springs which weren't designed for a shifting load that water causes? If you design it like you're thinking and you eventually get in a collision (it's not an "accident when there is negligence involved), don't think you may get a ticket cause you'd be visiting a jail instead. You better rethink your plan....