sounds like the coax might have a dead short in it
OR the PL-259 connector has a short in it.
See if you can get a mag mount antenna to try.
73

Gary Chenier wrote:

I purchased a used motorhome that has a factory installed CB. It's
lousy! Can't transmit or receive more that a half mile. I have tried
to tune the SW on the antenna but no matter if I lengthen or shorten
it the SW stays pegged at the top of the arc. I installed a new, thru
the glass mount antenna, and the same thing happened. I just brought
home a new radio and tried it with both antennas, but no luck.

Any suggestions?