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Whereas 12.6 v represents 100% charge (after being allowed to sit for a few hours since charging), 11.9 v represents 0% charge. Your 11.5 v tells me the battery is virtually dead. However, the cause is the issue, so lets look into this...
Usually, when a battery is bad, the charging (engine running) voltage tends to be higher. That your engine-running voltage is around 13.5 suggests (although not a confident indicator) that your problem is elsewhere.
First, of course, check your cables for corrosion -- that might be the reason the battery isn't charging higher.
Beyond that, you want to consider an abnormal battery drain. I assume you have a VOM (digital or analog). To start the search, remove the negative cable (always disconnect the negative cable first, and reattach the negative cable last) and put your VOM probes between the negative cable and the negative battery post.
The normal battery drain for a stock 1990 240 is 12 milliamps (0.012 amps) with brief pulses to 25 ma (.025 amps) to drive the clock's motor. 2.8 ma of that 7.8 is for the radio's presets, so if you changed the radio, these figures would be different. And if you swapped out your clock for a large tach without installing the small 2" clock instead, the normal drain is a steady 7.8 ma (0.0078 amps).
If you've got more amperage than that, you have to search further. Restore the negative cable connection to the battery, and then go to the fusebox. For each fuse, pulling them one at a time, put your VOM probes across each set of contacts and search for the abnormal battery drain. You should find the problematic circuit; and then you have to check all the components of that circuit.
Folks will offer suggestions: e.g., the electric door locks is a common problem, for example. Consider these as possibilities.
After you've found the problem, you have to check your battery to see how much it's been damaged by the severe discharge (batteries are severely damaged, e.g., sulfation, when their voltage drops below 50% for extended periods).
The best way is by measuring the specific gravity of each cell (you can buy a tester in an automotive supply store) -- 1.265 is ideally fully charged, whereas 1.100 is completely depleted. Remember to fill, if necessary (and then recharge before testing), with distilled water.
If you don't check specific gravity, you can check each cell for voltage. Pull all the caps and then use your VOM to measure individual pairs of cells. Start with a voltage reading between the positive (or negative) terminal and that end's cell by sticking one probe on the terminal and then the other probe inserted in the adjacent cell's solution (but not deep enough to touch the plates in the cell). Then progress with probes dipped into each adjacent pair of cells, successively progressing toward the opposite battery terminal. Each pair of cells ideally should show 2.1 volts at full health; and 1.99 volts is "dead"; but remember that the battery terminal-cell pairing at each end should be roughly half that. This assumes, of course, pretty good accuracy of your VOM.
Good luck.
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