No Power from house batteries in class C Four winds 2004 ?

Look for an "Aux Battery" switch in a panel with other switches and power and water level LED indicators. There may be a push button switch in the battery compartment that actuates a solenoid which makes a "clunk" when turned on.Let us know the manufacturer name, model name and floorplan info. Somebody may be familiar with that exact rig. If the mfr is still in business, give them a call.

Check voltage at the fuses. They should have the same voltage as the battery ( 12.6 volts) from both contacts on the fuse to ground. If not the problem is between the battery and the fuses, including ground wires.

We have a battery disconnect switch at the door on our Adventurer on 2004 E350. It looks the same as the switches for outside and inside ceiling lights. The disconnect switch failed. I managed to fix it - just a poor connection I think. Some RVs have a relay but ours is just a plain switch on a wire from the battery positive to a connection point near the converter and fuses.

My batteries have in-line fuses as shown with batteries removed. If you have the same, maybe yours have blown.

Are your friends batteries 6 or 12 volt? If six volt are they wired in series?

Did they accidentally reverse wire the batteries by putting the black wire on the negative post instead of on the positive? Check the converter for a blown reverse polarity fuse. the fuse might be located behind the converter.

Is the electrical outlet (30amp) plugged into the electrical box at the rear? I've done this many times with my Ford 2004 Four Winds. There is no disconnect switch on this model (at least never found one in 15 years).

check the ground of the house battery make sure its good then follow the positive wire back to the fuse panel and see where you loose power

check for.....
battery disconnect switch near the entry door. Must be on.
2 reverse polarity fuses on Converter. 30 or 40 amp.
1 inline reset-able fuse within 6' feet of battery.
fully charged battery.

Battery disable/disconnect switches are almost always discussed in the owner's manual. Look in the Electrical section of the manual.
If you don't have one, google the exact year/make/model and downloqd one from the manufacturer's website, or there are other manual download sites out there.

There may be an automatic circuit breaker in the OEM battery compartment. They do, eventually, die.

I would be surprised at a 2004 not having a battery disconnect switch.

Is it the Ford Chassis?