I assume you mean CB antenna -
The best locations are probably the ones that look the worst. A few ideas come to mind:
1) Put a rack (Yakima/Thule/homemade/etc) on the cab and mount the antennae near each end of the rack; the rack needs to be grounded.
2) Mount an antenna on the fifth wheel, run the coax down to the hitch where the trailer lights hookup, and couple it to an extension that runs from there into the cab. One more connection to hook/unhook. May not work well unless you have a metal roof.
3) Use a couple of magnet mounts on the cab.
4) Mount them at the front of the bed, on the bed rails, near the cab. You can get creative and mount a metal bar to extend them out from the bed; the wider they are the better the signal will be and the bigger chance of catching them on something (like your head, or mail box, or gas pump). Can also mount them on the crossbar of a head-ache rack.
5) Use a couple of thru-hole mounts and mount them on the front fender. Might be distracting to your vision, but at least you'll know if you knock one off.
You can find something wrong with all of these suggestions, so pick the one that is least problematic for you. I'm sure others will have suggestions as well.
Probably just as important to pay attention to good connections, good coax, good antennae, etc.