Bevman-
I can offer quick reply to keep thread alive until someone else responds with more details...
The
12V system (battery/charger-power supply/lighting/water pump etc) and 110V system (A/C or refrigerator power and outlets) should be linked at Univolt or Power Supply/Charger unit.. If they're missing in yours, a previous owner elected to make mods which limited the usefulness... Battery acts as reservoir for electrical charge, as Univolt power uneven, so both are required for working
12V system. Your battery and charger location are trailer mdoel dependent, and others hopefully can explain where they were.. You get to choose where they will be installed... Basic or cheap battery OK, but prone to faster charge loss and shorter life... It's a life choice you get to make...