I use a mount for my dodge conversion van that was for angled hoods. It fit inbetween the fender and the hood. Can be seen here:
VICTORY VSSSMV-1L S/S LEFT HOOD GROOVE MOUNT FOR VANS OR ANGELED HOOD
they also make this type that is adjustable:
The angle bracket works well on the conversion van with a short (3 footer) firestick antenna.
I tried one of those thru the glass antennas on the last rv and promptly tossed it out. NO good. That rig was a class A and I ended up mounting a bracket to the driver door frame and had the top loaded firestick sticking about 6 inches over the roof line.
On my latest chevy chassis class c I just used a straight cb bar bracket mounted on the base of the bumper on the drivers side with my old 7 ft francis whip. It is not as good as a dual mount but it does what I want, which is mainly monitoring and occasional chat. I put a tennis ball near the base of the antenna so it is a snug fit against the bumper and keeps the antenna from moving around too much. It does not clear the top of the coach but my reception seems to be quite good and much better than I thought it would be.
On all of the coaches I didn't want the top of antenna too far above the roof line. With the class a I even ran into problems with lower branches and had to take off the antenna sometimes.
I am a relic from the past and still run a pearce simpson tiger 40 radio which is almost as good as anything I've found out there aside from the top galaxy or cobra. That anl switch is a good thing on a radio. That is on the better units, the one on the cheap uniden (which I have and don't use) is worthless.
I made a mount on the ceiling out of oak for the new chevy. It is attached to a channel in the ceiling ( long self drilling screws thru the headliner to hold the oak board up) inbetween the visors. I like it up there better than underneath in the dash area. If you want a picture of the setup just yell. I'm not sure how the nav. is set up or if you could do that with it.