Using a gas powered truck to pull that much weight isn't really a safety issue. It's a question of how fast do you want to accelerate and how fast do you want to go up hill?
There may be some SRW 3500 trucks that are rated for that much weight but I would want a dually. I must assume you're talking about a FW if it's 15,000 pounds.
The RAM 6.4 Hemi would be my pick of the gas engines right now. You can even get it in a 4500 or 5500 cab and chassis if you want to really upgrade the payload from a regular 3500 pick-up.