I think that twice a month should be the norm
and not once a month. But looking at forums
from other carriers like UPS Freight, it seems
they are lucky to get their truck washed once a year.
I think that if FedEx still values their public image
they will try to keep the equipment as clean as possible.
What makes it hard is places like California that do not
want soap and chemicals going down the drain
and make it much more expensive than it should be
to maintain these fleets clean using these outside vendors.