I have been doing my yearly maintenance on the coach, check your air cleaner if you find a 1 inch hole in the bottom you are missing the vacuator valve, this keeps water from entering the cleaner and lets out any water that is in the cleaner, if you need one the part# P149099 at most Peterbuilt Dealers,$ 2.65.
and, Bill, if you used a deflector like I have (and posted) Rear Air Deflector (http://beamalarm.com/Documents/rear_air_deflector.html) you would not get any water going in the intake at top of rear. That to me is a stupid set up as water and all the crap flying off roof flows into it!!
My camera stays cleaner for a longer time too when driving in rain.
JohnH