I appreciate all the comments. It looks like "excessive" voltage at the campground is a rare occurrence, but some of us have experienced it. It looks like there are devices out there to help boost voltage, but no easy answers for cutting voltage. One option is to move to another site perhaps on another loop. Of course, this is not always an option.
In terms of adaptations, I guess I could plug my cube heater into an extension cord and connect it directly to the post's 15 amp outlet. This allows me to continue using my Progressive EMS to protect the trailer, but bypassing the system to use my cube heater, coffee pot, etc. Of course these items are being exposed to higher voltage, but I would rather risk my coffee pot than my Casita's AC, water heater, etc.
I will continue to research if the are any ways to bleed off a few volts of electricity if it is running more than 10% above 120V.
Laura & Dean | '05 Casita 17' FD | '09 Kia Borrego Limited V8 2WD