Yes 12V might be a little low but I need another decimal point, because nominal full charged voltage is at least 12.6V and usually higher for testing soon after the engine is shut off if the battery is healthy.
There are three main possibilities. The first is bad battery which seems less likely after trying a 2nd battery, so I'm ignoring that for now but if all else fails, revisit that in case your walmart automotive department is crap and lets their batteries sit drained on the shelf and that's the problem.
2nd more likely is you have an alternator problem. I would monitor the system voltage with engine going, wait a couple minutes after starting it in case the battery was low, and see if it's raising system voltage to around 14.(n)V. If it is not, then the alternator is a primary suspect, assuming (and do check this) that you don't have battery cable/terminal, or ground corrosion causing power loss.
3rd, especially if this problem is more prominent after it sits parked with engine off, is you have an excessive parasitic drain. On my '98, the normal parasitic drain in the fully off and sleeping long term is about 25mA, but is closer to 250mA for the first 45 minutes or so until it goes to sleep.
In that case, grab a multimeter and then there are a few options for tracing that. One is putting the multimeter in series with the battery and clamp, remembering it needs to be off and no wakeup events for roughly 45 minutes, like no opening doors (door latch switch cycling) or ignition cycling, or another option keeping it asleep as described, is a DC ammeter clamp on the battery cable, or another option is measuring for voltage drop across fuses to see if any particular subcircuit is drawing more than it should. There are various online resources for fuse type and rating vs voltage drop, mA multimeter measurements. I don't know which source is the best online for that, I have some saved locally but here is an example:
I would also run the engine for a while and then shut it off, measure battery voltage, wait an hour and measure it again, and if all else fails, repeat this process with the alternator charge wire disconnected once engine is off, and compare the result in case the alternator has leaky diode(s) and that is draining the battery.
Another option is keep the battery disconnected from vehicle positive, put it on a charger, get it up to full charge voltage, disconnect it and wait an hour to see how far the voltage drops. If it's much below 12.6V then the battery is bad. Remember that if your charging system is bad, that even though you have tried a new battery, it might be exhausted by a bad alternator quickly, or parasitic drain too but that typically takes a bit longer. Some auto parts stores can test alternators.