Sounds like you only have a single 12v battery?
Even if you have an inverter or converter or whatever it's called to make 120v power out of 12v power (unlikely, as those are only "standard" in higher end rigs), it would have died sooner than 24 hours, unless you have a whole bank of batteries, solar or some other source of power to recharge them.
There is a lot of other draws on the battery. The propane detector, and now they also have the O2 detectors hard wired in. The fridge uses 12v, too.
If you had that single light on all day, and it was your standard 12v bulb, it could drain quite a bit too.
Live and learn your rig is all you can do.. The furnace would use a ton of 12v power if you used it at all.
If it's not enough power, you have to upgrade the 12v system to accomodate your amp hour needs and go from there.
Good luck!
Mitch