I had the exact same issue while traveling in Canada a few years ago. Both phases were perfect during the day however about 8 PM the voltage of one phase dropped below 100 volts kicking my surge guard off. Luckily the 30 amp was not affected. Next night same thing, same time, I walked around the small campground to see if any large load was being applied and found nothing. My thoughts at the time made me think it was the electric company doing switching and possible low or bad transformer tap along the line
I had the exact same issue while traveling in Canada a few years ago. Both phases were perfect during the day however about 8 PM the voltage of one phase dropped below 100 volts kicking my surge guard off. Luckily the 30 amp was not affected. Next night same thing, same time, I walked around the small campground to see if any large load was being applied and found nothing. My thoughts at the time made me think it was the electric company doing switching and possible low or bad transformer tap along the line