I have the same problem on my '06. I took the coach to a truck stop CB shop in Federal Way, WA, that I have been going to for years with CBs, and they do an excellent job. They did the best they could with the factory antenna and cut it down in 1/4" increments until it was about 1/2 height, and it was the best they could do with their meters. It is better, but it's not good for more than about 3/4 mile range.
They traced the cable up to the antenna and removed the kinks in it, but it didn't help.
The CB shop mounted two of their best antennas and couldn't do much better than the factory, so we went back to factory antenna. They also said the same thing -- not enough ground plane with the aluminum bracket that holds the radio antenna and the CB antenna. Suspect it's the same reason I don't get much AM radio reception but do fine on FM.
Anyway, I think Mule Skinner's approach is the best if you want more range. On my boat I had a no ground plane antenna made by Shakespeare with the VHF and it worked fine. So I'm going to follow his advice and see if I can find a short boat antenna and try it.
Former Owner, 2006 36MDDS
2010 Arctic Fox 22GK Travel Trailer
2007 Toyota Tundra