I think you are trying to have "on-Board" lighting even when the tow vehicle is miles away.
I added small sockets and 12 volt bulbs to both my porch lights,(1157 tail light bulbs) both my inside lights (a couple clearance light socket and bulb) and the galley( it was a exit light with 12 volt backup lights already in it).
From the onboard battery I went to a main fuse and manual shut off then to a second fuse block with the cheap little glass fuses. From there to the lights. By adding a tow charge line to the battery you can maintain the battery while towing and use it for many many hours independent of the tow vehicle. I never ran the charge line I simply charge the battery before the trip. For giggles I took a tap-a-light ( one of those cheap little battery puck lights) and retrofitted it with 120 volt christmas bulb a 12 volt clearance bulb and socket so I have an AC/DC and self contained battery light. Use your imagination and a little hot glue and you can have any kind of 12 volt light you want.
Both of these are 120 ac and 12 volt DC
