Don't forget to take into account pin weight, which is the amount carried by the 5th wheel hitch in the bed. In your case that would amount to about 2,400 lbs.. Now subtract the weight you're carrying in the bed, 2,400 lbs., from the GVWR of 12,300 lbs, and you arrive at about 9900 lbs. that is actually being carried on the axles. So if your axles are rated for 10,400 lbs. you're good. Not great by any means, but trailer builders generally try to get by with the least they can, and still stay legal.
Going to larger axles/wheels/brakes is not a bad idea, but don't do it because you think you're overweight, do it because you want bigger tires, bigger brakes, and better handling.
