There are a couple of possibilities. 1 is the one that CW sold/sells for a glass window mount. the other is to use a marine coax cable which has a built in ground wire. Of course it will entail using a mount on the outside. The window one would work if you have a high side window to adhere to. Again may need to add a ground plane to the roof (metal) to have better range.
Its a shame IMO that people no longer use the cb. Have seen many cases with rv's in particular where there is a problem and you cannot get the driver/passenger attention to it. There was a forest fire some years ago where a guys towed had a flat, the tire disintegrated and sparks were coming from the wheel. His message, 'I thought they were just mad at me for going too slow and holding up traffic'! Open compartments, dragging chains, etc. I carry a hand held but it has no range but it does work if close enough.