THIS video from Facebook shows the potential of not doing the calculations and for ignoring Science.
Distance from axle to tool box could be a serious issue when you consider how levers work.
I have also seen some Class-C Motorhomes with platforms attached to the trailer hitch receiver. Some have mobility scooters on them some tool boxes and some even have motorcycles. These items might be 100# but I am afraid that the total might approach 500# in some applications. While the hitch and platform might be rated for 500# or more the back end of the RV may have lower limits.
Here we see a 5,000# hitch on an RV with only a 250# load limit.
It is important to remember that just as with axles, springs, and tires, the load capacity is limited by the weakest link or component.For axles, the weakest component is many times the tires. You may have an axle rated GAWR of 3,000 and the tires may only be rated for 1,510# but swapping the axle with a 4,000# unit will not raise the GAWR unless you also change the tires and probably also the springs, brakes and spring mounts too.
This same situation can occur for the hitch. Class-C Motorhomes are many times built on the "Cutaway" frames of standard vans. But the RV company extends the wheelbase by cutting the rear of the frame than welding in a couple pieces of steel to make it longer. This does not result in a longer but equal strength structure. In fact, it may mean the total frame strength is now lower.
Back to the question of what happens even if you don't have load limitations of the frame or receiver? Hanging weight off the rear bumper does TWO things. First, It obviously increases the load on the rear tires. Second it decreases the front tire loads and transfers that load onto the rear axle and tires. You need to be sure that this addition is not overloading any of your tires.