Michael,
Divide your scale weights by 2 per axle, then use the dual numbers for the drive and single numbers for the tag and steer. Here's an example:
Steer - 15,340/2=7,670 Use the single weight from tire chart
Drive - 19,580/2=9,790 Use the dual weight from the tire chart
Tag - 15,060/2=7,530 Use the single weight from the tire chart
I know nothing about Firestones, but be sure your not exceeding the tire pressures for the weight. Look at the max pressure numbers on the tires. Also remember, your tire pressures are going to really increase when they get hot, so it's important that you don't put in too much air when cold.
On your way home, stop at exit 306 and re-weigh at the Q-T Truckstop. If they're not busy, you can weigh each wheel position by moving off the scale with each side of the bus. The concrete apron is wide enough to weigh one side then circle around and weigh the other side. Make sure someone is outside helping so your sure to split the axles evenly and not get bad readings. On any of the scales, you can screw up and not get the axles split evenly on each independent scale platform. Also, are you sure your toad didn't end up on your tag axle scale ticket? Always unhook the toad.