That surface-mount glass-tube fuse block appears to be something added by a previous owner. I say owner-added due to the way the wires run and the fact that electrical contacts are exposed. (Is there a cover missing?).
If possible, I'd contact the previous owner to ask why the fuse block is there.
If that fails, you could try tracing wires to figure it out or perhaps pulling a fuse at a time to see what's affected, if anything. I'm not familiar enough with wire colors to interpret anything from what I'm seeing.
I would have guessed it's there because of something wrong with the original DC panel but that guess doesn't square with your observation that it doesn't appear to have power to it.
The other thing I'd try to do is identify that box the fuse-block is mounted on. I've been trying to find a similar photo and at this point I'd guess it's a Progressive Dynamics unit, perhaps an AC/DC distribution panel (only) or perhaps one with a battery charger built into it. I'd look for the model number. If it is indeed a Progressive Dynamics model,
here's a link to the owners manuals for discontinued Progressive Dynamics products. which could be useful.
As Bigskyxj says, your furnace-fan motor is 12-volt. The furnace-fan should run whether your camper is on battery-only or is plugged in to shore power. If that's not the case, I'd suspect the converter/charger isn't handling the switchover properly.
Also-- curiosity question-- is the yellow box a spare-fuses box? I ask because it looks like one but it seems out of scale and it's odd to see one on what I assume is a vertical surface.
PS- If any of you 80s-vintage owners recognize the unit in question, please chime in or send me a PM.
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