This should be in it's own story.
"doctech"?
Vector is good about their warranty. Call them and send it back. The replacement unit may not be the same one you returned.
Remove the 4 corner screws from the front. Look for a small short on the rear of the sockets. Sometimes a tiny short will cause that.
Check the battery connections, from the battery, to the clamp's connection to the cable, to corrosion inside the cable, to the inverter. That tends to be an intermittant problem for larger (over 100W) loads.
If you can not return it under warranty, remove the board and look for a single strand of wire going from somewhere to somewhere else it should not be going.
After that, buy a new one. Save the reciept and packaging.
G-