Lots of stuff going on here, so do a search for the myriad problems/causes with these batteries. First, what you are seeing is absolutely normal with this battery, same as the Odyssey 2150. Normal is not the same a desirable, but this is the way these batteries behave. You Nat Luna monitor is not accurate (I have three of them) but is what we would call "directionally correct." The full charge you show after the short drive is just a surface charge and the reading you get after a few hours more accurately reflects the actual state of charge. If you are using that battery in your Tacoma, Toyotas seem to generate lower alternator output voltages than other makes, and that contributes to the battery problem. The Odyssey needs 14.7V at high amperage to achieve and maintain a full charge, and you truck cannot provide that. My truck can't either, and I have a 270 amp alternator. I am not aware of any normal production vehicle that can produce the current necessary to keep that battery healthy over the long term. From the day you installed it, it began to sulphate, and the sulphation is a vicious cycle, especially on a vehicle that is not driven much. Sulphation prevents the battery from accepting a full charge and that inability to take a full charge gets progressively worse as the battery ages. UNLESS you tweak your charging system to deliver 14.7V (not practical in most cases) and drive it enough with minimum alternator output of 50 amps or better (so, idling for three hours to charge it does not help). The alternative is to acquire an Odyssey-approved charger to maintain that specific battery. Odyssey has a chart of approved chargers on their website under Literature or Tech Support tabs. There are not many suitable chargers, and the Odyssey Ultimizer Charger in the 40amp and 50amp versions is out of production. Odyssey is bringing out a new line of charger, but the big ones (the one you need) won't be out until spring. Per Odyssey, you need a minimum of 25amps at 14.7V+, up to 15.6V for desulphation. Odyssey says 25amps is the minimum, but 40amps is better and 50amps is ideal. But the charger also has to have the appropriate charge profile for that battery. If you can find one of the Ultimizer 40 or 50amp units, those would work but they are big and heavy, made by Schumacher for Odyssey. The easiest charger to find right now is probably the Ctek US 25000, which is "only" 25amps, but is on the Odyssey approved list. Once you get the battery back into condition (there is a recondition/desulphation cycle on the approved chargers), do a full charge on the shore power every eight weeks or so, whether or not you think you need it. I am on my fourth 2150/PM1 in about seven years and I am now being religious about maintaining that sucker. If you can't get it back to a full charge (12.84V or higher OCV after a eight-hour rest following the charge cycle), try to get Sears to warranty it. Sears will resist, so tell them to charge it on their eqpt and get it back up to spec. Make sure they do the voltage reading after it has rested for at least eight hours or, preferably, overnight. If it's not at 12.84 OCV or higher, it's not at full charge. Odyssey also has a percent of charge table on their website. Probably a good idea to take a copy with you when you go back to Sears. Be aware that this is a marine battery, and the warranty is only three years, compared to four year on the automotive version which is the same battery without the marine terminals.
Now, aren't you glad you asked?