gwcisme, WELCOME to iRV2!!
I would say it definitely depends on "where" this unit is mounted as to whether you actually want the unit to do both the NAV and the rear/side cams...
In my brand new Essex for instance, I chose to mount a second 7" screen over on the left side of the dash,in line with the left pillar in order to see the camera images without physically turning my head some thirty degrees and down to the right. I really felt I was taking my eyes off the road...I still use the NAV/DVD/ portion of the radio but it doesn't mean I take my eyes off the road.
I have the Pioneer AVIC Z1 in my coach, Newmar also uses the Pioneer AVIC D2...they both are NAV/camera monitor adaptable and are nice units.