Lance trailer “flat roof” makes roof water ponds -- problems

Lake effects on flat roofs is a big reason rv trailer/house trailer mfg moved to the crowned truss type ribs in the roof many years ago. I've seen water standing 1"+ deep around the A/C units especially on older trailers that the roof sagged with time.
Having been in the rv/mobilhome assy/repair side I'm surprised to hear any mfg would use that old system.
Maintaining them, especially with the old galvanized steel and later aluminum roofing material was time consuming if we wanted a the unit to last long term.