I also have a 2500HD Duramax. I also pull a 30 foot 5th wheel. I have taken it across the scales. The rear axle came in 4700 lbs. With 245/75-16 load range E tires, it can safely carry 6084 lbs. The rear axle is rated for 10800lbs. The frame is the same as a 1 ton dually.
Your load limitation is the tires, not the capacity of the truck.
I still have 1384 lbs available.
I have an '02 Duramax since new. Other than replace batteries I have not had any other repairs except for injectors. In 111,000 miles replaced 2 and a half sets of injectors. The last set is of the current Bosch design.
When that happens, I put 4x4 block under each leg and then pull the pin and drop the legs onto the blocks. Then remove the blocks and lower the legs. This will allow both legs to hit the ground at the same time. The idea is to keep the weight equal on each leg. After you unhook you should have enough adjustment to level the trailer.
The number of axles determines the amount of toll. After crossing the state line into Oklahoma continue till you get to the toll booth.
Wlhen you pay the toll, they will charge you for entire trip to Tulsa. Keep your receipt and when you exit at Big Cabin you will be given a refund for the difference between Big Cabin and Tulsa.
Towing a trailer the toll will be about $5.00, if i remember correctly.