You need to get in touch with the Ford zone service manager for that region and plead your case. But you say it's for a friend so let him do it. There may be a sympathizer at the Dealer who could find his/her number for you?
The motorhome definitely has a chassis based GCWR on a sticker usually above the drivers seat. The calculation is the same for all vehicles "Actual" MH weight combined with the "Actual" toad weight must be less than the GCWR period. "Actual" means weighed at a scale when loaded for travel.
1993 firmware really?
Pretty sure it's an analog electric and vacuum based system. There should be a vacuum storage tank under the hood, these used to rot out and could cause this type of issue. Good luck.