Good performing antenna for 5er
You might try making your own L-brackets, and place them on each side of the top forward portion of the 5er.
Use a standard bi-phase harness down to the pickup bed.
Get a female barrel connection and add some good quality coax from the CB rig to the bi-phase harness. The length of the jumper coax should be a multiple of 1/2 wave length which for most coax is 0. 67 times the open air wave length. 1/4 wave whips are 108", so a half-wave is 216" inches times 0. 67 equals 144" or 12 feet. Just right!
Don't worry about the extra length of coax. This is almost DC since it is at 27 mHz.
Use some standard short whips up there with a max height of about 13' 6" and you will enjoy so really good coverage for a mobile installation that is pulling a solid RF shield behind them.
Be sure to bond the brackets to the trailer skin/frame.