On most Class C MH you will find that the ground plane is the problem.
All the fiberglass in the coach area is not a ground plane.
Some people use non-ground plane antenna such as the firesticks on the side, back or mirrors (if you have metal mirrors for the brackets).
Be aware of your antenna overall height.
I went with a left front fender mount and a 4' fiberglass antenna.
I am using the Everhardt STT Super Tiger Full Wave CB Antenna.
http://www.wearecb.com/everhardt-stt-super-tiger-full-wave-cb-antenna.html
Some will say that the left front is not ideal.
That is true.
There is no ideal place on a fiberglass Class C.
The from steel hood and fenders give me a ground plane.
Non-ground plane antennas, from what I can read do not have the
gain of a regular antenna. Also their coax length is sensitive
to length,as that forms their ground plane.
So far the performance has been very good.
SWR 1:1 to 1:3.
I am using a Cobra 29 NW.
I can hear others very well and I get reports that others can hear me
very well. That is what counts.
It came in very handy on a recent 3 week trip.
I85 outside of Montgomery, Al. was closed for 48 hours due to a semi accident and fire. We were detoured on small county roads for 3 hours.
Lots of information was exchanged on the CB.
Those without one were clueless.