I've never used anything other than the phone's built-in antenna. After speaking to several folks on this and other forums, I'm not convinced supplemental antennas work. And at more than $500.00 for the one listed in the link you attached, it seems like an expensive experiment.
If you haven't tried this yet...Ask someone who uses a different cell company to test their signal in and out of your coach. Then try it with someone who uses your carrier but has a different model phone. Could be a clue.
I've had phones that work better or worse than others. Right now, my wife and I have different phones (hers: LG - mine: Motorola) but both with the same carrier. We've been in locations where she had a signal and I didn't, and vice versa.