I contacted Forest River and Atwood about this issue, and received replies from both.
I'm told the inverted louvers are intentionally designed that way to prevent the furnace from sucking in hot 'exhaust' (i.e. CO) air from the chrome 'chimney' below into the furnace's fresh air. This could extinguish the furnace flame. Installed correctly by the coach builder, any water entry vie the furnace's panel louvers should drain out harmlessly via furnace drain holes and not harm furnace components or get into the coach itself.
Also, Atwood told me the panel 'gasket' must be intact, installed and fitted correctly. A missing or improperly fitted gasket could allow water entry into the left side of the compartment where damage can occur.
As regards my Prism's AFMD furnace, I believe this is no longer a furnace issue. I note that my furnace chimney pipe is bent downward to prevent water entry there.
Kudo's to Forest River and Atwood for their prompt replies.