The cat scale folks don't care what you weigh. Having the weight ticket (showing "in limits") could get you out of one if stopped "for looking overloaded."
For a 5th wheel it is easy.
1) Loaded as you would for camping, with a full tank of gas and your family aboard...
2) Pull up and onto scale pulling all the way forward placing your front wheels on the small leading scale plate.
3) Get out and make sure your rear axle is on the second plate and the camper's wheels are on the third plate.
4) Use a yard stick to reach up and push the "press to talk" button (or call them with your cell phone) to get your initial weight and tell them you will need a second "reweigh."
5) Drive to the parking area and drop the camper.
6) Drive back onto the scale and putting the truck exactly where it was before; press the button and get your second weight.
7) Go hook up your camper and go to the shack to get your weight tickets.
8) the first weigh is 10 bucks; the second is 1 dollar.
treat your family to dinner and figure out the data.
My tickets for a previous camping trip are attached.
If you subtract the truck alone weight from the combined weight you will get your true camper weight. (Compare to the camper's GVWR to make sure the camper is not overloaded)
Add the truck axles (connected) together and subtract the disconnected truck weight to get your true pin weight.
Add the truck axles (loaded) and compare to your trucks max gross weight (GVWR) to make sure the truck is not overloaded.
Compare the connected combined weight to your truck's maximum combined weight (GCWR) to make sure your truck can pull the combination.