Welcome to iRV2.
First post I see.
EDIT: You didn't say if your coach is plugged in when this is happening, so I assumed it wasn't.
When your batteries are low enough to set of a low volts alarm , it would take 8 to 10 hours of driving on the highway for the engine alternator to fully recharge them. So seeing them above 12 volts after a half hour is only a surface charge, and the reason they are low again in days.
JMHO; Remove the batteries from the coach and charge them for 48 hours, let them sit for 12 hours after charging and have them load tested. That's the best way to check for a bad battery.
If they test ok , then put them back in the coach , but don't re-connect them till you're ready to use the RV.
More info on batteries and charging on this site