Each axle stands on it's own merits. Each can carry 3000 pounds max. If one is carrying 2,500 pounds great, but the other carrying 3,500 pounds is overloaded while the total is still 6,000 pounds on a scale pad that supports both trailer axles at the same time.
That is why if getting close to the maximum load capacity, one should go to scales where you put each axle on separate weigh panel and get a reading. The trailer will need to be level.
I acquired individual wheel scales (two sets of four scales) and was able to see what each tire is carrying. The left-right weight distribution is important as well.
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TV - 2012 Dodge 2500 4x4 Cummins HO, automatic, Centramatics, Kelderman level ride airbag suspension, bed shell
2014 31' Classic model 30 twin beds, 50 amp service, 900 watt solar system, Centramatics, Tuson TPMS, disc brakes, 16" tires & wheels