The RV starts and runs, and the vehicle's headlights and the internal lights associated with the E350 Ford van on which the RV is built all work. That is, everything running from the vehicle battery is fine, but nothing running on "hotel" power (the deep-cycle RV battery that is separate from the vehicle battery) works. Earlier that afternoon, I'd had occasion to use the water pump, which runs on the same circuit, and it had worked at the time, but not now.
This is not a catastrophe, but it is an annoyance. I don't run the RV's refrigerator. I take my showers at the office. I even have a reserve battery for my mobile phone. The toilet works even if the water won't run. (It's better if I can put some water in the system, though.) It's not cold enough to need to use the furnace. The main annoyance is that I can't run my CPAP machine.
I keep flashlights handy, and used them to make out the bed, and then I called Lisa. She walked me through checking the RV fuses, all of which are sound. That probably means that the main fuse under the hood leading from the "hotel" battery to the fuse box in the back has blown, but I couldn't easily check this at night, so I thanked her for her help and got what sleep that I could.
Even in daylight, it's not easy to check that fuse. Not that it's in a difficult location; the two batteries are in the forward engine compartment under the hood. Except that due to a weak spring, it's very difficult for one person to open the hood of the RV. You need two people: one pushing down on the hood, and another pulling on the hood-release lever. Once I can get someone to help me, I can look to see if that fuse is the problem. If so, it shouldn't be too difficult to replace, although I'll need someone to help me again after I get a replacement fuse.
It is physically possible to run a 12V extension from the plug in the forward part of the RV (that runs off the vehicle battery, not the RV battery) to the CPAP. However, while it seems unlikely, I'd rather not risk running down the RV's engine battery overnight running the CPAP and leaving me unable to start the RV at 4 AM when my alarm goes off.
Even if I'm unable to fix the problem with the RV "hotel" power, I can still get by this week. It just will be less convenient.