1984 pace arrow 12 volt system

[boringlarry]

Hi, I'm new to the MH world, and I have a 12 volt problem:
When I disconnect my 110 volt line from outside, my 12v system seems to only partially work, ie my house lights wont work, but the refrigerator brain still does. (also, 12v fridge light seems to be running off the house batteries, as it's goes dim unless i keep the those batteries charged, which i've been doing with a charger, and, recently I have heard the fan running from the converter, and it seems like i haven't had to plug in the charger lately....)
I've checked all fuses and breakers everywhere and all are good/on.
Any thoughts are appreciated!
...and also, on the bed pedestal is an outlet that has horizontal slots ( - - ) ...what could that be for?

[94-Newmar]

Typically those outlets with 2 horizontal slots were used for 220v window air conditioners, but from a completely different era than your 84 model motorhome - and they've been long outlawed in the US. Did you try a volt meter on the outlet? It would be extremely rare to have 220v available to you in the RV.

From the chart below if you're describing it correctly, it could be a NEMA 2-15 or 6-15 if it has a grounding pin, both rated for 250v.

NEMA Plug & Receptacle Configurations Reference Chart (Straight Blade)

Either way, both appear to have been used for 220v applications. Very strange you'd have one. If it's really connected back to your power panel I'd start doing some serious backtracing to see where/how it's connected to your main panel. Finding any hacks a previous owner has done might explain your other voltage oddities you describe as well.

[Gary RVRoamer]

All your 12v circuits in the "house" should be connected to the house batteries, and the converter/charger should keep those charged while on shore power. It sounds as though some of your circuits are hooked to the converter output, but not to the batteries. Strange. Would have to trace wires to figure why/how that could be.