I questioned the directionality issues with the CB shop. I used to install CB's years ago and we always used mirror mounts on trucks. I got into a heavy discussion with the shop's installer because this RV has Powder Coat on the mirror arms. He thinks we could scrape through that but the swivel of the mirror bracket would cause grounding issues. I don't know why he thinks that,
the swivel core is a large bolt.
So he suggested the 102 on the rear bumper. I think the RV body would kill most of the forward reliability of the antenna. They claim as long as three inches is above the box, I should be fine. Not from what I remember unless antenna technology has improved light years.
I would not even consider a metal plate on the roof. I already shudder when I go under some bridges. I don't want to lose the antenna before I get a chance to use it.
I used to run a 102 bumper mount on my Pickup, but it had full clearance above the tailgate. No box in the way. I personally would prefer a mirror mount setup.
i think the 102 may be more hassle than it is worth.