I was trying to find some info on this as well . I have been looking at truckers forums.
Personally im using a magnet mount Wilson on my trucks hood. It receives ok and seems to transmit well within a mile. Therefore it works well when I travel with an additional vehicle, convoy style.
If your looking to reach out in a 360 further.....maybe the hood mount magnet won't be up to the job.
On the camper roof rack might be a good place. Use a good cable .
If you need more length. Try to avoid an in-line connector , use a single length wire .
this was taken from a truckers forum :
"""" a few items need to be addressed here as a professional installer .....
1) steel OR fiber door ? .... ant needs to be grounded .
2) 18' of " Real Coaxle " is recommended for premium performance .
3) again , there is usually a rubber gasket separating your mirror hardware from the actual door surface which is considered " ungrounded".
Remove some gasket ,die grind to metal or ground your mount .
4) Stick with a WILSON antennae."""""