I saw this reply by mike_mcginn on RV net:
"The Sprinter dome light is only wired for the
driver & passenger doors. The contacts on the
side and back doors are for door open warnings.
So no dome light for the rear and side doors.
In the cargo version there are optional lights
that could be ordered from Dodge but this does
not apply to our conversions."
These battery powered LED Lights from Sylvania might be useful:
I did read that they rattle a bit but a small piece of tape or dab of silicone would probably quieten them.