I have a 96 Dodge 3500 w/ a 12 valve Cummins in it. I've insatalled a Bully Dog turbo back exhaust, K&N air filter and a # 8 torque plate( 210HP-570torque). I now pull a 5th wheel that weighs about 12,000 loaded. I have no problems getting down the road and average about 13mpg pulling. Any thoughts if I'm going to have problems if I upgrade to a 5th wheel that weighs around 15,000 loaded.

You need to check the towing capacity of the 96 3500. Your mods will not affect this number. I think 15000 is about capacity for your unit. Not sure if the capacity will be on your vehicle but a Chrysler dealer should have it. Don't take their verbal response...look for it in print.

Shouldn't have any problems pulling. How about stopping ? Do you feel comfortable with the ability to stop the 12,000 lbs ? What affect will 3000 additional lbs have on your stopping ability. If you have an exhaust brake, you'll do OK. If you have some trouble now getting the 12,000 lbs stop and you don't have an exhaust brake. You may want to add one if you go to the 15,000 lb 5th wheel. That was my problem, the truck pulled the heavier 5th wheel without any noticeable difference, but stopping that was a different story. My new truck, stops the 5th wheel on a dime, a very long dime, but it does a great job at stopping. It's a very secure feeling.