Not sure what the article said but here is what most use a reference standard for battery condition (approx 75 deg F average):
This is with engine OFF and after the surface charge has been bleed off (For toad - Lights on for 1 minute).
If the converter, inverter/charger, solar, or engine is running you will be reading the output of the charging device, not the condition of the battery. That table shows a battery at rest (no loads connected, no charge device ON)
A toad voltage of 12.4 (with key ON) after several hours of being pulled behind the coach would be normal if there is no charging connection to the toad from your coach.