You can check your tires and get a close estimate of the most you 'should' load on the axles. If you have 8 lug 16" rims with 3,042 pound rated tires, then you probably also have 6,000 pound rated axles. Otherwise 5,000 pound or less would be installed with 15" rims and lesser tires.
Personally I would order a new RV with disk brakes and heavier axles than what the trailer will require. Just so that my 3,000 pound rated tires will only see around 2,600 pounds on them! Or if the trailer must have 3,000 pounds on each tire, I would want 17.5" rims with a 4,000 pound rated tire and rim. .
If all else fails, you can call this company, they can tell you where to look for the model number on your trailer axles, and they can tell you a model number XYZ is rated at so many pounds. .
ETrailer.comGood luck,
Fred.