We load and unload so often, I worked on coming up with something reliable and easy several years ago. These pictures are on the old flatbed, but the new one is exactly the same.
Because we do this so often, I installed angle iron on three sides of the camper as rub rails.
We have angle screwed to the flatbed. The original design, because the angle couldn't be permanent due to other uses, just had two drop pins in it. Either way, these are the guides on the flatbed.
Loading is a breeze; no shifting. Same spot every time.