This is a typical Block Diagram of your 30AMP Electrical Wiring for a RV Trailer...
The RV Trailer idea is to have all of your basic needs items (Lights - Fans - 12VDC Controls etc) running from 12VDC and the Appliances type nice to have items (Air Conditioner-Converter/charger-Fridge-Special lights- etc) running from 120VAC.
All of the 120VAC and 12VDC wiring is originated from a Power Distribution Panel where the 120VAC Circuit Breakers and 12VDC Fuses are located. This Distribution Panel is fed by the 120VAC from the Shore Power Connection and 12VDC from the Battery Bank.
When Connected to Shore Power a 120VAC CONVERTER/CHARGER unit is used to produce 12VDC to power up the 12VDC side of the Power Distribution Panel so that all of the 12VDC items will operate when plugged into 120VAC Shore Power. This converter/charger unit is also used as a battery charger to keep your connected batteries charged up when you are operating on shore power.
When camping without Shore Power then the on-board battery setup supplies only 12VDC to the Power Distribution Panel and will only operate the basic needs items 12VDC lights and some selected 12VDC items that are basic things you need to live in your trailer when camping off the power grid. Of course alot of other things run off of Propane that will work when you are on Shore Power or just the Battery.
Some folks will add an optional INVERTER (Not a converter) which will operate from the trailer battery setup and provide 120VAC to operate some of the selected 120VAC Appliances that only operate when you plugged into Shore Power.
There actually some thought going into the trailer to make it work at both Shore Power locations and basic needs when camping off the power grid.
It always amazing to see folks use "I guess this works like this" - There are many manuals out there for study on how your RV Trailer works. Be a good idea to read up on them...
It does take good planning on your part and the only way out of many experiences is having some good PLAN Bs in place. Might help you out when it gets dark on you at 10PM at night when camping off the power grid.
just for info