Questions about tire and axle loading

Yeah you are pretty much right. But don't forget that the camper itself is not all you need to allow for. There are passengers and all the stuff you put in your camper. Usual allowance is 1000lbs. It is quite true that ratings are usually conservative, how conservative you never know, but part of that is to allow for smacking the occasional speed bump or pothole or other roughness, it's there for a reason. It's best not to exceed your payload rating even if many people do. Still you will have plenty to choose from, just don't expect double slide outs and 11 or 12 feet of length.