I have a customer with a rv with currently 3 cameras a rev. a left and a right, all of which activate through their repective signal..... however the customer wants to ugrade to the f90 and i am stuck trying to figure out a switching system to incorp. his cameras which still work, however his montior doesnt ..... any help will be greatly appreciated
Edit all...
THIS IS WHAT YOU WANT and IT runs off of 12volts!!!
That will work but it is not necessary.
Use a a three position dial or toggle switch, RCA Y connectors, and three relays. Join all three cameras into a single input on the head unit. Then wire the switch to the relays so that each camera only has power when it is selected on the switch. I have done this on at least a dozen times and it works perfectly.
A little late with my reply, but I just stumbled across this item:
This will let you use the F90s rearview camera input with his existing cameras.
A bit pricey, but RV owners tend to have deep pockets... 
-Eric