1980 Autocar Returns to the Road after many many Years!

This reminds me of working on my old 92 Peterbilt 379. In the cab, everything sounds and looks the same. The low air pressure alert also sounded the same.
As a former truck driver, I have an idea: Attach some blind spot mirrors to the two poles protruding from the front bumper. Mirrors will be extremely useful for changing lanes and, in some cases, backing up your rig in tight spots. By the way, I'm referring to large blind spot mirrors.
The 5th wheel is probably debris on the sliders.
Remove the plate, turn it over, and you'll have easy access to the latch mechanism, which, like the hubs, will benefit from a good clean and lubricant.
BTW, be gentler when backing up to the trailer or that hitch pin will shear; they don't take much stress before giving way.