If you happen to be talking about the ceiling lights in the sleeping area and bathroom (assuming for the moment that you have a rear bath model), they all run on the purple wire that runs to the device called a "Univolt".
If you have the model I described above, your Univolt may be found as follows: facing the curbside closet between the bathroom and the sleeping area, open the closet door and get down on your knees.
W-a-a-a-y back on the floor of the closet in the right-hand corner (yes, your
right hand) you will see a little door. That's the door of the Univolt that leads to its fuse panel. Pull it down to open.
The purple wire is on the right side of the fuse panel (you DID remember to bring a nice bright flashlight with you, right?) and it's fuse may well be blown. It may also be that the clips holding the fuse in are corroded and not conducting electiricity well enough. This can mean that the purple wire can heat up, thus causing the fuse to blow again.
Now, this next bit may seem to way far ahead of what you want to hear, but it's very likely that when all is said and done, you really need to just pull out the Univolt and replace it with a more modern three-stage charger/converter that will NOT constantly hum the Navy Hymn (as do most old Univolts), will NOT boil your battery, but will provide quiet, dependable 12V
power to your lovely trailer's lights and pumps.
I will look around and see if I have a scan of the wiring diagram, but really, there are only eight fuses in the Univolt fuse panel, so have a look at that first.
Here's the diagrams:
Wiring Diagram for the big models (up to 31') from the 1977 Service Manual. The only difference I can see is that there are two batteries and they are up front.
Image of the front of the Univolt: the fuse panel (bottom) and bonus image of the Command Panel