Which tour are you going on? I remember feeling there was no way I could carry-on only. Five trips later, it's routine.
Make a detailed packing list/checklist. Test pack, edit packing list, test pack, edit again, repeat until you meet the airlines requirements. Be sure you can lift your suitcase into the luggage racks, tow it over cobble stones at least 1/2 mile and up/down stairs. There is no hard, fast rule that you need to do carry-on only on a RS tour, but you will thank yourself that you did.
I travel for almost a month with carryon only. Everything, well almost, I take is sink-washable; synthetics dry overnight. I made a mistake last year adding a cotton sweater because I like the looks, but it took three days to dry after it got too nasty looking to wear. Oops! Jackets I usually don't wash for the same reason. Cashmere sweaters dry overnight or by day 2. I do laundry every two days or on the first night of a two night stay. I take plastic folding hangers to drip dry shirts -- no wrinkles. Most tours have arrangements with a hotel mid-way send out laundry. I've never used the service.
For a few years I extended my suitcase on the way home to stuff it full of what was in my tote and checked it. The carry-on tote held souvenirs. That was before United delayed my luggage home. I don't like that kind of annoyance. This fall I just shipped the largish souvenirs home (from Deruta pottery on Village Italy tour). Everything else was flat and didn't take much room so I carried on all luggage.
Enjoy your trip!