antenna placement advice?

I recently purchased the viper coil antenna, which i have no complaints about , i run a galaxy 959 i (get out good) and my recieve is excellent but with it mounted to my mirror im havin trouble nailing tree branches and in one case a low bridge. i was wondering if i mount it to the back of my sleeper on the horizontal grab bar if i would still get the quality out of it that im getting now? I measured it, the coil will be below the roof but the whip will still be above by atleast 2 feet. I tried to tip it forward on my mirror but i didnt like the angle of the coil because it was almost flat, i guess i was wondering if any one has tried this b4? as far as grounding i was going to ground it to the exhaust bar. also i have it mounted on the drivers side mirror now and still hit tree branches, but i thought i would seek advice b4 i try to run the cable through my truck.