Nice to hear from a fellow CB'er.
I'm not a techy but this is my practical experience.
I have successfully mounted my magnetic Wilson 1000 antenna with it's 5ft. whip to the center of my TV roof and have had great success.
Not sure that mounting on the roof of your 5th wheel will give much assistance as it's roof is rubberized, galvanized etc.
When mounting on the TV I) mount directly on the roof, not bracketed to an aluminum roof rack II) center to the roof of the TV as the TV roof helps increase your reception.
Last and unfortunately todays white noise is not conducive to CB reception so your distance tends to be affected.
Today's CB antennas no longer need to be adjusted for gain... so I'm told?
Hope this helps
GPG