I'm no electrician, but if you have 118 V. from black to ground at the receptacle, but only 80 v. from black to neutral at the receptacle, I would have to believe the problem would have to be in the neutral wire. Is there any way you can check the resistance in the neutral wire to see if that could be the problem. I would also check to see if one of the other outlets in the area of the microwave could be on the same circuit by shutting off the microwave breaker and see if any other outlet is dead. If so, it could be a connection at that outlet. That could be one of those problems that can only be found by the process of eliminating all the possibilities no matter how remote.
Roger & Cheryl Full Time since 2002, RVing since 1975 2008 Chevy 3500 Crew Cab Dually, 2012 RW 36RL