Been a while but had the same issue with a previous employer. If memory serves there is a stepped spring on one end of the mirror arms that can have the holding tension increased by tightening the nut below it. It was a workshop job at the time not least because the company didn’t allow drivers to meddle with vehicle maintenance.
Are the trucks brand new? If so it doesn’t reflect well on the dealers pre delivery inspection does it.
Maybe poster Norb can advise have you tried a pm to him?