There is two layers of fuses between the Converter/Charger and the battery terminals. One is IN-LINE very close to the battery terminal. This can also be a resettable circuit breaker on the newer trailer....
The next fused layer is called the REVERSE POLARITY FUSES which are located at the 12VDC Power Distribution areas. These are large VALUE fuses...
About the only that makes these fuses blow is actually connecting the BATTERIES in the wrong POLARITY...
Also some trailers have a DISCONNECT BATTERY SWITCH installed between the battery terminals and the 12VDC Power Distribution panel.
Its easy to check what may be at fault. You have already indicated that the ceiling lights work when plugged into shore power. This tells you the on-board converter/charger unit is functioning and is connected.
The next step is start at the battery terminals using a DC MULTIMETER and start looking at connections between the battery terminals and the 12VDC Power Distribution Panel. Something is disconnected if your battery read 12.6VDC with the MULTIMETER at their terminals.
Also make sure the battery polarity is configured correctly before replace any blown fuses you find. Other they will just blow again. Hand trace the battery case marked NEG BATTERY TERMINAL and make sure this cable goes directly to the TRAILER FRAME GROUND which will be very close to the battery terminals.
Roy and Carolyn
I claim Horse Creek Country in Southern Ill - Momabear is from North Texas
We live in King George VA
RETIRED DOD DOAF DON CONTRACTOR Electronics Tech 42YRS
"We're burning daylight" - John Wayne
2008 STARCRAFT 14RT OFF-ROAD POPUP with PD9260C and three 85AH 12VDC batteries
2010 F150 FX4 5.4 GAS with 3.73 gears - Super Cab - Towing Package - 2KW Honda EU2000i Gen
K9PHT (since 1957) 146.52Mhz
"We always have a PLAN B"