12.1 VDC indicates a battery already at 50% discharge state. Since you have been plugged into shore power for three days, obviously something is preventing your RV charger from charging the batteries. Do you have a Battery Disconnect switch inadvertently activated? If not, you need to diagnose why the charger is not charging the batteries..... You just installed them? I'd also be suspicious that you did not hook up all the battery cables properly.....maybe you missed one? One thing for sure.....get a charger on those new batteries quickly before they get further discharged and ruined!