Here is a link to Tredit wheel who supplies the wheels to Keystone:
http://www.tredittire.com/Aluminum-Wheels-C10.aspx
Find your style wheels and click on the picture and it will tell you the spec's for that wheel.
If you look at the axles, it should have a sticker on them, if not, its imprinted into the axle also, just hard to see.
My 2010 3150RL has 6000# axles, don't know if they have changed for 2013 though.