I'm going to refrain from really getting into the weeds here! The Freightliner Cascadia requires a somewhat different antenna mounting situation for a decent CB setup.
View attachment 131363
This is the simplest way to mount a decent antenna on a newer Cascadia.
Mount-
CASP4 Freightliner Cascadia CB Antenna Mount for 2018+ P4 Models
Antenna-
Skipshooter S6 6 5000 Watt Fiberglass CB Antenna - BLACK
The reason for the tilt is to get the coil in the top part of the antenna away from the cab to minimize reflect (better SWR). A good antenna has to be in free space as much as possible.
You can substitute a 4 foot Firestick for the 6 foot Skipshooter. Skipshooters are generally not available in truck stops. The mount can be had in some truck stops. The most important consideration is the antenna should be "top loaded". No base load/center load coily looking antennas.
The built in antenna on the Cascadia is worthless, but it won't blow up you're radio.
For some help routeing your coax-
I normally wouldn't recommend truck stop coax, but you seem to be just starting out so it will work fine.
As far as the background noise...............we are on the upswing of a skip cycle! Radio signals tend to travel much further bouncing off the atmosphere during a skip cycle. Not much you can do about it. Now if you're getting interference from the truck that's another matter. The most common sources are cellphone/tablet chargers, inverters, and aftermarket LED lights. I have found that charging off of USB ports built into the truck don't cause much of a problem. Wall warts and cig lighter chargers do!
This should help you get going without breaking the bank! A good antenna will make even a basic CB radio do a fine job.
JD