Washers ??
Guess Ive never heard of the washer needing replaced. Just the grease seal and bearing sets.
I gotta' ask if you have ever changed out a set of trailer bearings. You need the right tools for driving/pressing out the bearing races and then driving/pressing the new races back in. Install the new race or removing the old ruined bearing race incorrectly can ruin the hub.
The grease seal is soft and can't damage the hub if it going in a bit crooked. Bearing races are harder than the hub and can remove material if done improperly.
I R&R my tandem RV trailer axle bearings/brakes/all suspension parts each spring. That way I know they will last all summer till time to winterize. The trailer in my sig has over 140k miles and replaced the seals three times since '97. I hand grease the bearing and never use the zerks on any of my trailers bearings.