Well, I hate to say it cause the dang thing weighs about 20 pounds but (read the fine manual) (Read The Fine Manual)
The manual SHOULD tell you a few things, including where your battery or batteries are located and (And this is the biggie)
WHERE THE BATTERY DISCONNECT CONTROL IS..
Now, they put 'em in several places.. On many motor homes and trailers there may be a switch panel just inside the door, it will often have momentary switches, 3 position (on/off/on) with springs to hold them in the "off" position. On many motor homes it will be on the dash, It can also be just about anywhere. On many trailers it's in the battery compartment itself.
(Which means locating the batteries..... Check to see if one of the entry steps comes up, that's a common place)
Other places are basement compartments and any closet or cubbord at FLOOR LEVEL, they almost never put 'em higher than the floor.
But that's going to be my best suggestion.. NOTE the batteries may be flat and need charging (just leave 'em connected with the rig plugged in for a day to do that)
Next. ONCE you find the batteries, if you can't find a disconnect, you will need to start tracking power.. I find a test light is the best for this.. To make a test light (I like home made better than store bought) take something like a license plate lamp socket or a marker lamp light socket and matching bulb, Put a clip on one of the leads and on the other one solder and tape on a longer flexible wire. Put a "probe" on the other end (A small ice pick type probe works well) or you can put on an insulated clip
You start by hooking to the battery No light = dead battery or bad lamp. Then move the ground clip from the battery negative to the chassis, (NOTE: stop at the first NO LIGHT and fix it before continuing) NO light = bad ground, check both ends of the cable.
Then follow the battery postive cable, each device you come to check both terminals NO LIGHT =
A: Bad connection (Correct)
B: Open switch (Close) or relay (Find the control and operate)
3: Failed fuse or circuit breaker (Reset or replace)
It's that simple
I do not think you will get to the test lamp stage though I think your batteries are in "Store" mode (Disconnected)
A volt meter can be used instead of the test lamp.. but it is harder, much much harder, since the meter has to be read where as the lamp you just glance at it.