Charging System, Moderately High Voltage, need ideas

ALL Monaco/HR built motorhomes charge BOTH battery banks from the Engine Alternator. Regardless of year. The fact that you have verified yours does NOT, means nothing except your system is malfunctioning. Find and fix the dual charging problem also. IF your APU (Genset) was running during the voltage spike, that may be your problem, Was your Genset running? Doug