When you come through toll booth on PA turnpike you get weighed, If I had over 1/2 tank of fuel in my dually (no trailer), it would give me a class 2 ticket, which is a higher toll. Then when paying toll at the exit booth, I would tell the toll taker and they would only charge me the class 1 price saying my weight was around the 9500 lb that would give me that Class 2 ticket. Also to slow down on the scale at the booth to get accurate reading, hard braking on it gives a heavier reading.
PA follows Federal DOT rules. Under 26000 Class C license is only required, but I got fined for no medical card since I was over 17,000 lbs combined. 10,000 truck, 14,000 trailer (25,000 total). Also was questioned about what was inside trailer, and got the lecture on inter vs intra state commerce, DOT numbers, those fines, etc...