Several years ago I bought two Cobra hand-helds (HTs, in Ham parlance, which is easier to type

) to use for caravanning. I had a setup that I thought worked pretty well, at least as far as CB goes. Not as good as a couple of cheap FRS radios (which we use now), but for a combo handheld/mobile CB setup like you described, here are some suggestions:
Make sure the HT has a 12V power-in receptacle. The batteries in a CB HT last about 2 min.
As mentioned, handheld CBs won’t transmit out of the cab, so make sure the rubber ducky is removable and you can get an adapter to connect a real external antenna.
The units I had also had combo speaker/mics available as an accessory. As a mic it’s nice because holding up a handheld that has a stiff antenna cable and power cable hanging off of it is a real pain. The speaker part is nice because you can hold it up to your ear if there is a lot of ambient noise. I found that if I clip the speaker/mic to the seat belt I could hear it well, and it was handy for talking into.
With the speaker/mic, 12v power, and mag-mount antennas the setup worked pretty well. Matter of fact, since the handhelds TX the full legal power, the setup worked as well as any <legal> mobile. But I got really tired of all the idiots on CB that live to be obnoxious, so actually quit using them several years before I started using the FRS radios
I may have the stuff in a box in the attic (minus the mag-mount antennas, which were trash to begin with), if it didn’t go in a yard sale. And I seem to recall that one of the two HTs quit working, but if you think it might be worth the cost of shipping from TX (78613), I’ll dig around.