First thing I'd do is make sure your guage, sending unit & their interfaces are working properly.
One of the first things I do with ANY vehicle I buy (including my bus) is to get a couple of jerry cans full of gas, fill up the gas tank, write down the starting milage, and run the tank dry so I KNOW where empty is, and roughly how many miles I can go until I will run out. I usually mix highway & city miles to try to get a more realistic idea.
Once I have determined how many miles I can go on one tank, and average how many miles per gallon the vehicle gets, I can drive without fear of running out by allowing myself a 50 mile buffer, i.e. if I get 300 miles per tank, I start looking for gas as I near 250 miles travelled.
Hope this helps, although I'm sure some one in this forum probably knows what size tank was installed when your bus went down the assembly line.
~(G)Q Arduously Avoiding Assimilation