I will opine..
I have had many--many trucks, still have alot too.. Payload is not the end all number.. Trucks are routinely under rated because they know people will over load them. This is a fact! Do you think for a minute if your trucks payload is 2000lbs. and you put 2010lbs in it it will break? Nope.. The rate them for safety--sure, but then everyone drives different and when tested they run them on a safe course and call in people to test it in all situations... For those on here that roll down the road with a trailer doing over 55mph, most do 80+ that I have found don't know a friggin hill a beans what they are talking about! Besides, in most states the speed limit for a vehicle with a trailer is 10mph under the posted limit, NOT 80 mph!
A Cummins motor will pull more than you think. I have had two 2500's and a 3500 dually, all pull the weight just fine, the 3500 is just more sure footed due to the rear tires. I also only run commercial tires, I see over 100k miles on them and they are rated higher than the tires that come stock, not those ******* huge tires, be it mud tires or the all terrain not to mention the offsets that make your truck look like a skateboard, both are stupid, unless you have a 4x4 for that is street driven only I would not recommend that setup if your going to tow.. A gas engine is fine, until you start climbing hills, then the diesel really shines. You will not see the fuel efficiency from a gas engine like a diesel either so you made a good choice. I will sum it up here, a diesel is made for this, a gas engine IS NOT.. Although a gas engine performs ok, it cannot match the torque, efficiency or longevity you will get with a diesel, if they could you would see Kenworths, Peterbuilts all running the road with gas engines...