I've used CAT scales at truck stops for years. Drive onto the scale and pull up until your front axle is on the first pad, the drive axle is on the second pad, and the trailer axles are on the third pad.
The intercom button will be way up in the air above you. I open the driver's window and stand on the window sill to reach the intercom button. Mash the button and wait until someone says something to you. Tell them you are a private RV, truck number 1, name whatever. After a few seconds they will tell you they got it.
Then drive off the scale and park in the truck parking lot. Go into the store and get in line at the fuel desk. Tell the clerk you have a scale ticket for an RV, truck number one. The clerk will retrieve your ticket and tell you how much you owe. Probably $10 or so. Pay the clerk and you can go.
If you need to reweigh, maybe without the trailer, explain to the clerk. They will tell you how to go about it - and the reweigh cost is usually only $1.00.
Remember that the truckers behind you wanting to use the scale are in a hurry, so don't mess around when you are on the scale. Do your business, and get off the scale.
There are other threads on IRV2 that explain how to fabricate a long "arm" to reach the intercom button without standing on the window sill of the truck's door. I don't use an arm because I'm agile enough to climb up and stand on the window sill to mash the intercom button.