I'm a mechanical engineer, a tinkerer, a fabricator, and a farm boy, so I know exactly where you're headed...
Maybe in an EXTREME case, you could end up with over 5000lbs...
Just off the top of my head, I'm estimating it would take a tongue weight of around 2000lbs on the extension with no weight distribution to achieve that.
The truck would be undriveable with so little weight on the front wheels. I'd surely notice right away when I couldn't turn.
The joke's on you though: Even with 5000lbs on the rear axle, I'd still be under the truck's RAWR of 9350, and well under the tire limits of >10,000lbs.
