I've looked at this thread many times. I keep thinking someone will bring up the other side of the safety issue, because too little tongue weight is bad, but too much is just as bad in a completely different way.
There is a lot of good information on this thread. However, many posts are missing a few facts that should not be overlooked. 10-15% tongue weight is ideal, but should never exceed the trucks rating OR the HITCH RATING. Most factory receiver hitches until around 2011 were very cheap/weak and not rated for nearly the towing capacity of the tow vehicle. Many since 2011 are still cheap/weak, but are at least rated for a reasonable weight for the vehicles they are mounted on. My 2006 Chevy 2500HD with a Duramax had a factory hitch rated at only 7500lbs and 750lbs of tongue weight. A normal 16' car trailer with a mid size SUV will likely weight over 6000lbs. On my 16' car trailer, with a 2010 Acadia centered (front to rear) hung over the rear just a touch, had 1000lbs of tongue weight (16.7% tongue weight)(as indicated by my weigh safe ball mount). That is good for relieving the stress on the trailer axles, and would pull very nice down the highway, but would have in no way been safe for the trucks factory hitch, nor the trailer itself. My trailer does have a slight bow between the coupler and axles, showing that at one point it was over it's capacity for tongue weight. I watched one hitch ripped off a truck, and have seen the aftermath of a few others. The one I witnessed lost the entire hitch entering a parking lot, when it hit the dip the trailer just continued down, safety chains were still attached to the hitch and did nothing to control the trailer. The car trailer it was towing had a pickup on that was even with the front of the trailer and the truck sagged before it lost the hitch. I suspect tongue weight was the main problem, but also likely wasn't the first time it was loaded like that.
Do not rely on the hitch to be strong enough. We also have a 2013 Ram 3500 with a cummins. There is no rating stamped, stickered, or posted anywhere of the hitches weight rating, only the trucks towing ability. All anyone says is Class 5, but that only means the weight rating is over 12000 lbs. Most hitches have a tongue weight rating of ONLY 10% of the rated capacity. The only actual numbers I could find for the Ram, show 16000lb WR with 1600 TWR, and I can not verify that anywhere. But if I tried to pull my 14k trailer, fully loaded, with a 15% tongue weight that is suggested by many, I would be over the tongue weight of any factory hitch I've seen at 2100lbs. While searching for a weight rating of the Ram factory hitch, I found several posts showing hitches that were ripped off the trucks. Looking at my Ram, I can say the hitch is no where near as robust, or robustly attached to the truck, as other heavy duty hitches I've installed that have been rated for 1800lb tongue weights, and not even in the same league as the one I installed with a 2700lb tongue rating. It is on par with the 7500lb hitch I removed from my duramax. However, it is not braced by the bumper that is separately mounted to the truck frame like the chevy, rather the bumper is only attached to the hitch, which is only attached to the frame on the last 6 inches, much the way the bumper is attached to my chevy.
Be very careful not to overload. In an ideal world, the towing vehicle hitch would be located very near (or over) the rear axle, and capable of significantly more tongue weight. Because, ideally all trailers would have 20-40% tongue weight, but that is not how trucks are made or how trailers are designed. They would be much safer to tow that way, similar to a big tractor-trailer. Design specs for most trailers only allow for 10% tongue weight.
Air bags or timbrens only help the ride and looks, they do not increase the towing or carrying capacity. They can really help smooth out a rough ride and stop the squatting dog look. I have had timbrens, and both my duramax's have air bags. Headlights should be adjusted with the trailer loaded and attached to the truck. Weight distribution hitches are great for putting some weight back on the front axle to allow the truck to handle much better for a much safer ride, but truck and hitch capacities must still be followed.
In my opinion a lot of trailers are made with the axles too far back. Much more so in recent years. Partly because car and equipment trailers need to tow well empty as well as loaded, which means they need extra tongue weight when empty to offset the heavy ramps and bracing behind the axles. But I believe mostly it is due to customer demand, ie. people buying trailers that pull easier, therefore with more tongue weight. It can be terrifying when a trailer gets into a sway swing. The general public doesn't know and doesn't care how to properly load a trailer. A trailer evenly loaded with axles back further will likely have about 10% tongue weight. However, with a front heavy pickup or tractor, they can't be loaded forward and still maintain safe weight distribution.
I highly recommend a ball mount with a built in weight scale. Especially with my airbags, it can be really easy to overload the hitches tongue weight rating. A good friend of mine who worked for John Deere for 30 years liked to have the ball drop 2" with tongue weight. That is all.