When I first set up my inverter, I had a blinking light issue too. Those yellow lights can be tricky sometimes. As for the charging, if the battery meter shows full, it might be that it’s done charging but still needs some time for everything to settle. Give it a bit longer.About the 27.0V reading, it’s normal for 12V batteries in series to show that if they’re not fully charged or there's some voltage drop. You can check with a converter like this one mV to V to make sure you're not missing anything.