This is part of the reason we encourage the use of GMRS and ham as the antennas are shorter and can be mounted higher and in places a CB antenna can't. They also don't require tuning, have better audio, and aren't as susceptible to RFI.
that is not entirely true , sorry
if he has an rtt he will have most likely interference issues with the mount and antenna itself no matter what
Ive never had an amateur antenna I didnt have to tune ever so slightly to be honest
run the longest antenna you can, keep it high & in the clear, it needs to be properly grounded as well
not much metal on jeeps for an image plane
yes ham vhf, uhf , gmrs are far shorter antennas but also need the same attention
Ive had really good luck lately with a hood (lip) mounted NMO antenna, but jeep hoods might not work as well unless youre not afraid to drill a hole
keep it as far forward and in the clear
73