You are correct in that there is little to no mention of the laundry loads as a symbol of specific significance to the overall plot in Death of a Salesman.
However, Death of a Salesman is a play about self-realization, in which more than one character finds out certain truths about themselves. When you think about it, the story "brings out the dirty laundry" of the Loman family into the open. Who tries to downplay everything as if nothing is really going on? Linda does. She consistently tries to clean the dirty laundry of the family.
From the very beginning we notice how Linda is subservient to Willy, how she consistently takes his side, and how she openly blames Happy and Biff for upsetting Willy. Every time there is a slight chance of reconciliation between Biff and Willy, in comes Linda to make it look like everything is running smoothly in the family.
Therefore we could argue that the laundry load is a symbol for the reality of the relationships in the Loman Family.