Galaxy DX-959 40 Channel CB and what CB Antenna ?

You must use the correct coaxial cable for CB. Do not use TV or satellite cable- it is the wrong impedance. Most antennas come with a length of the correct cable- but if not,or if you need a longer length it is readily available at any dealer. Putting the proper connectors on, if necessary, takes some skill.

Don't use a magnet mount antenna, even on your "metal plate", on a fiberglass class A. A no-ground-plane antenna is required. I really like Wilson and K40 antennas but I'm not sure if they make a no-ground-plane model.
Whatever you choose, be sure to tune it properly for minimum SWR. If you don't, transmitting thru it can burn up the finals in the radio. You will need an SWR meter to tune it. They are pretty inexpensive at truck stops or Radio Shack.

With fiberglass rv's you have to get a no ground plane antenna. They have them at radio shack. I ran my cable underneath and came out a hatch to the side mirror.

Might have to get creative.

Good Luck.

Well I forgot to mention I have a class A motorhome, however there is a metal plate on top where old dish was but not sure about the cable going thru roof and down to dash area, the dish coaxial cable is there can this be used or do I need a CB coaxial cable..

My cable was run down the back window then under the cab of the truck, then through
the engine firewall to my CB. Personally I always have the installation done at the CB shop and then can alson tune the antenna for better reception.

Well I forgot to mention I have a class A motorhome, however there is a metal plate on top where old dish was but not sure about the cable going thru roof and down to dash area, the dish coaxial cable is there can this be used or do I need a CB coaxial cable..

Looking at buying a Galaxy DX-959 40 Channel AM/SSB Mobile CB Radio with Frequency Counter, but what CB antenna is best for a motorhome and best place to mount.

Wilson Antenna is what I use and it is the magnet base that is placed on center of my cab. The Wison antenna will also unscrew with just your hand, which is good when you drive into low ceiling parking structures i.e. Airports.