when does Ford powertrain warranty starts ?

The chassis should have been shipped to the motorhome manufacturer as an incomplete vehicle, and once the motorhome was built, then often a new VIN is assigned to the motorhome for registration purposes. The chassis manufacturer will continue to use their VIN for maintenance tracking and warranty issues. This is the key part and germane to your problem: Once the motorhome is sold, the dealer should notify the manufacturer who will submit or the dealer himself will submit a delayed start warranty notification to the chassis manufacturer. This apparently did not happen so the chassis manufacturer has to rely on their sale date as the warranty start date.

To correct the problem, contact the motorhome manufacturer and insist they intercede on your behalf with a delayed warranty start date notification to the chassis manufacturer.