GVWR (lbs/kgs) 9000 / 4082.4
If your gross is 9000 pounds, then you will have about 1170 on the tongue (13 percent of gross). Be sure to get spring bars that are up to that amount. That leaves 7830 or so on the axles. If your axles are 4400 pound axles you have 8800 pounds axle capacity and the math aproximately says that leaves around 1000 pounds for all your stuff and water. Water is 8.3 pounds per gallon.
It becomes a little more than just changing the axles. Tires and springs are an issue as well. You will need to be able to have the carrying capacity in the tires to match the new axles, wheels to accommodate the new tires/pressures and probably upgraded springs.