Tow rating is actually directly related to the number of passengers AND cargo in the vehicle. It isn't only due to the RAR (rear axle rating- though you have to watch this also!) but related to the GCWR (gross combined weight rating= vehicle + trailer).
GCWR- actual vehicle weight= max trailer weight. Until the new tow rating standard was followed recently, the manufactures would rate a vehicle's tow rating with only the driver and no cargo to get the max tow rating.
GVWR is the total weight the vehicle is rated for including tw (tongue weight) as stated. So one has to be mindful of all the manufactures ratings, and while not usually an issue, don't forget the FAR (front axle rating) either.
ALSO, remember to subtract any and all accessories you add to your vehicle! Bed liner, tonneau cover/truck topper, step bars, etc. Anything that would not have been included in/ on the vehicle when it was weighed as it rolled off the assembly line (since '06 or so each vehicle has a yellow sticker in the drivers door jamb. "Passengers and cargo not to exceed "XXXX"lbs.)
TW can range from 10-15% depending on the trailer and it's design. Open trailer are lower, enclosed and travel trailers a bit higher, though again depending how it was designed. If you are having an issue towing a trailer, weighing it is the best way to find out if you have an issue or not. BUT, you may need to in crease tw to make that trailer a little more stable even if the tw # looks good. I have a travel trailer (tt) that at 12.6% tw it wasn't 100% stable. While I haven't reweighed it yet, I moved some things around to increase the tw and it is stable now.
I have learned a lot, but am still learning! I used to be in the "Oh I have "XXXXX"lbs tow rating, I'm good" crowd! Main reason we have a 2500HD now as we were over our GVWR by up to 200lbs (still under RAR by a little) with our 1500.
And as Meburdick replied about air bags, they can help, but a lot of times they are not used right. Most times a wd (weight distribution) hitch is needed, not air baags.