You still must have an issue or a parasitic draw somehow. I have a 12v battery I removed when I did my new battery bank over 2 weeks ago. Sitting there in the garage it still tests at 12.5v
Check the water in the battery and make sure it's above the plates in each cell. If your going to do some boondocking your going to want to nail down what's drawing amperage out of your battery and when it's drawing it. You'll damage that battery letting it get that low and shorten it's charging capacity and lifespan. Buying another battery and wiring them together to double your amp hours or chucking that one and doing a pair of 6v will do wonders for your system if you plan on doing more boondocking.
The "Golden Rule" is to never let your batteries go below 50% or damage occurs
1999 Suburban 2500 4x4, 2016 Jayflight 23RB Elite, Fiberglass, Polar Package