When I first got my enclosed trailer I spent some time scheming how I could use onboard batteries to remove my reliance on paddock power. That is to say, I could run everything on paddock power if available, and on big batteries if not. Here’s some notes on what to buy and mistakes to avoid.
2 large 12V batteries wired in parallel. Get batts designed for deep discharge.
Charger that comes on automatically. Look for an old school design, not something that requires hitting a button to be turned on. You want it to come on automatically when ever you connect to shore power.
Big inverter. At least 2500W. This won’t run an AC unit or a big air compressor, but it will run anything else.
Buy a half dozen GFI power outlets. and steel boxes for them. Put a pair at your work bench and another pair at the back of the trailer. In each pair, wire 1 set to shore power, and the other to your inverter. This will allow you to connect to 120AC no matter where you are in the trailer and whether or not you have shore power.
The wires from battery to inverter need to be HUGE. Plan for a capacity of ~200A to flow from batts to inverter. This means big fat wires and multiple ground connections to the frame of the trailer. If anything is going to cause you trouble, this is it. Every time you consider your wires to the batts and your grounds, think 200A.
Put in a big circuit breaker/ switch that can handle 200A or so. This will isolate and protect your batteries. Make it easily accessible so you can flip the switch and isolate your batts each time you put your trailer in storage. Otherwise your batts will go flat when the trailer is in storage and deep discharges like that are hard on batteries. Something inevitably drains your batts when the trailer is in storage. Maybe the inverter pulls some current when off, I dunno.
Connect the aux power from your trailer’s 7pin connector to the batts. This will allow your truck to charge the trailer batts while they are connected.
Connect a cigarette lighter thing to 12V. Those are always handy.
There are new fangled 120AC outlets that are also USB chargers. That would be cool.
You can bring a little $88 Harbor Freight generator to events. Small, light, and will power your fridge and charge your batts if no shore power. My rig will run my dorm fridge for only about half a day.
Be advised that inverters vary a lot in quality. Cheap ones produce square waves, not sine waves. Some electronics are sensitive to that. My cheap invertor won’t run most electric blankets, for example. So in addition to a regular electric banky in the trailer, I also have a 12V electric banky.
The pic below shows a switch and set of outlets at the back of the trailer’s workbench. The switch not only controls the power source for the outlet you see, but also the outlet at the back of the trailer.
There’s one more outlet in the trailer, a double set near the side-door. It has a set connected to shore power and set connected to the inverter as described above. The reason for that was that it seemed nicely flexible to have both power sources simultaneously available on some occasions.