There are many 2 in one systems where the front and interior are in the same unit, but i am not entirely sure they come as 2 channel ( front + interior ) as 2 channel systems most often mean front + rear and so adding a interior camera will make it a 3 channel system.
Also a few are not suited out of the box to mount on a vertical flat camper windscreen, so with those you often have to fabricate a wedge to go in between the system and the windscreen to space it out a little and angle it a little so the lens can film strait out the windscreen.
EDIT: Just looked up the camper and it is a sloped windscreen, so that should be no problem then, well aside for finding a system only front + interior.
The good systems today are made with Sony starvis 2 sensors, model numbers IMX 678 for 4K resolution and IMX675 for 1440p resolution
Your friend might also consider a regular 1 channel front dashcam for the driving part, and then parried with a CCTV camera for the interior, like Wyse and others make, nifty little CCTV cameras with battery or USB power and a memory card inside.