Hi...a bit to late probably but..
The alternator is the usual culprit with these trucks..a slipping belt or bad connection. Also a dirty foot controller pot can play havoc,usually after pressure washing etc.
Worst case scenarios are usually a few teeth missing off the ring gear or a wiring defect...or....a controller fault. The inverters usually fail completely,and smell very burnt!! Hope this will help your customer out.
Regards,
Simon