Ok, another Blob bringing up a subject I have seen on this and other RV sites.
A combo unit, 12 volt and 120 volt lighting fixture. An RV parts distributor did have a few that he had sold years ago. They were designed for custom rebuilds. RV manufactures didnt install them due to liability issues with people installing the wrong bulbs. (household style lightbulb, but 12 volt, into the 120 socket). They did have a combo unit on the market a few years back that had 120 fluorescent twin bulbs with 2 small automotive single element 12 volt bulbs.
Having a switch on each end.
I have made this concept work by building my own. Using a 12 inch twin bulb under counter fluorescent light fixture and putting in two 12 volt light sockets on either side of the mini transformer of the fluorescent light.
I used side marker light sockets, also called a 194 bulb and secured them to the frame housing of the light fixture and running the wiring oposite the 120 volt wiring, Using Red and Black to differentuate from cabin 120 volt wiring.
YES, you can build your own, or hope you can still find a retailer still selling a combination fixture. Just be careful and know what wires are what etc.