Like AquaMan said the capacity will be stamped on the axles or will have an information tag attached. Anything is better than the plastic bushings. My last trailer had me scratching my head trying to figure out why I had one tire wearing on the inside edge, my bushings were shot. I replaced them with the Dexter E-Z Flex system which cured the tire wear problem, provided a smoother ride as proven by the clothes hangers not falling in the closets during transit, and took away some of the noises heard during towing. I don't have any experience with the brand you describe but I looked up the website and feel these should be a good investment for you.
2015 Flagstaff Classic Super Lite 831FKBSS
2010 F250 Super Duty 5.4L