It should charge from the hook up cable using mains and also charge when driving. It is advised if not driving much to plug in once a month to top it off. The alternative is solar which is well covered on the forum.
At this voltage there is a chance the battery wont bounce back as the red areas on that graph indicate where the battery will accumulate damage.
I would assume that it isn't being charged and would require some investigation.
There are 2 batteries in an ocean and depending on your experience level of DIY should decide if you tackle it or take it to a garage.
First and easiest things to check are.
1) What happens on the screen with the engine running?
2) What happens on the screen with the hook up cable attached?