Comment on my antenna please.

cable length 50.3 meters??

terminate the end of the coax with a 50 ohm load and see what happens.

 

no comments from the experts?

Wow ... just wow.

 

Sorry, I am not an expert but here is my opinion. This sounds like the multiplex system from hell. Multiplexing different signals on one system - proof technology is not always the best solution. This is why more people are bypassing what the truck came with and installing their own 50 ohm coax between the antenna on the driver's door and the CB radio. The KISS method: simple, cheap and very effective fix.

 

Is your Nano vna still showing an impedance of 100 ohms? What's the reading for 27.205?

Edit: What radio and antenna are you using? Are you only talking and receiving a mile or two?

 

What's the RL (return loss) of your antenna system according to the Nano on channel 20?

 

Okay, that's not good. Your problem is due to a lack of RF ground. It's a common problem with today's trucks. Even though guys driving Volvo trucks claim the trucks have metal in the cabs, they still seem to have the same problem as other trucks have with their CB antenna system.
You need to add ground straps to the cab. It's known as bonding. Here's a great website that explains it.

Bonding

If this doesn't solve the problem, then I'm not sure what would.