My CB Radio has a lot of noise and no distance.?

I have a Cobra CB radio with a single antenna. I've tried different radios, different antennas, and new coax cable but with no difference in results. My CB has so much interference when I turn it on that the needle is about 1/2 way up on the RF meter and I only have a distance of 1/2 a mile. There is very little difference if the truck in turned off or running so it isn't engine noise. My SWR is under 1 and my antenna is tuned. What the heck is wrong? It has to be something in the Kenworth I'm installing it in. I have never been able to get any distance out of a CB radio/antenna even though I always tune and check everything. Please help me.