Unless your tires are really old there should be markings on the sidewall about recommended cold tire pressures for either single or dual use. Generally about 80 PSI would be right for bias ply 9.00 X 20's and 10.00 X 20's. Radials of these sizes will need about 95 to 100 PSI.
Hauling 2000 gal. of water on this size of truck can be really tricky....especially when you have a partial load, lots of sloshing around. Hope you have baffles in your tank.
James
