Laundry on 21 day BOE

This recent thread about the 14-day tour has some responses relevant to you: https://community.ricksteves.com/travel-forum/tours/laundry-and-light-packing.

We took a 14 day tour , our experience was that at least one or two hotels either have a laundry or are nearby a public one .

We visit Europe for 4-5 weeks at a time and can get by only using a laundrette 2-3 times , we bring clothes that are easy to sink wash .

You can do it any day you want in the sink of your room. I find that fits much better into a busy sightseeing schedule. Bring layers and fast drying fabrics. Have a great time!

We did this tour in spring of 2015. There were laundry facilities where you could do your own in our Austria hotel (Reutte). In Monterossa there was a place where you could drop off your laundry on your first day there and they had it sent to your room all packaged up the next day! This is something everyone will be interested in with such a long tour and I'm sure it will be discussed by your tour guide. Also, this information is usually mentioned in Rick's books too! You can always ask at the hotel's front desk, We had a guy on our tour that had his shirts laundered and pressed very neatly at several of our stops (they don't usually advertise this service). To get through in the meantime sink-washing is what most people do. You can use the hotel's shower gel or in our case we brought along some "laundry sheets" that disintegrated and acted as a soap. I brought a large heavy-duty ziplock bag that acted as a washing machine of sorts (thanks to Pam for this suggestion).......I also brought some collapsible hangers to dry our clothes using the shower rod. Some hotels will specifically ask you to NOT do sink washing....notably our Bacharach hotel. This was a very old hotel and I can't blame them for not wanting our clothes to drip on their old floors! You just have to be respectful and obviously wring out your clothes as much as possible. This will help them dry faster anyway. Your hair dryer can be helpful too.......and having "quick-dry" clothes will aid in this as well.

Just returned from the 21 day BOE in September. It was a fabulous tour. Never needed to use a laundry service or laundermat, though both were available throughout the trip. I washed clothing by hand when we arrived at our two-day stops. Rolling items in a towel to remove excess water then hanging them to dry completely worked out well. I used lightweight folding travel hangers and plastic over the door hooks to hang them on. Both items were purchased on Amazon. I took clothing and underwear that dried quickly. I tested each piece at home using this washing method. Clothing didn't make it to the suitcase if it didn't dry quickly. The one exception was socks. They can be worn a little damp or dried further with a hair dryer. Try to limit yourself to a smaller piece of luggage with no more than 20 pounds, and pack clothing that can be layered, as Rick Steves recommends. You will be glad you did. A sample packing list, along with other helpful information, can be found under Travel Tips on this website.

I would have posted the same thing that vickie.santos did. I have taken 14 RS tours, including the 21 day BoE, and spent time in Europe either before the tour or after the our, or both, and have never used a laundromat.
We have lots of ExOfficio clothes and underwear which we sink wash in the evening before retiring to bed and they are dry and ready to go in the morning. They are more expensive than others but have lasted us for 16 years of trips to Europe.