Determining kingpin weight?

While we are on the subject of weighing, someone mentioned an easy way to check your camper weight at a public scale, on your first weigh, weigh the truck and camper together. On your second pass (leaving the truck/camper as is) all you have to do is unlock the hitch and raise the landing gear until the hitch isnt contacting the plate anymore. I'm assuming until you can see daylight between them.

The second weigh will show a difference, and thats the camper weight?

I'll bet I'm missing something here.....

Ron W