This forum is a great resource for RS tours in particular. Thanks for all the great suggestions here, and here's what I found helped most on an RS Italy tour:
--number one above all: pack lightly. I packed four T-shirts, two pairs of capris, and one pair of jeans (jeans worn on all flights) and did fine with sink washing. The buses had to park several blocks away from the hotels, and I was very happy to have only a 20" rolling carryon and a small cross-body bag while going up and down curbs.
--rubber stopper for hotel sinks and clothesline. None of our hotels had drains that closed, so the stopper was key. I didn't mind sink washing, and in fact, it seemed it would have been hard to schedule a laundromat stop where we were located. In any case, I was glad to spend a few minutes most nights in the room rather than an afternoon in front of a washing machine.
--SmartWool socks; decent, broken-in walking shoes; and moleskin
--moneybelt, worn every day without fail, and in Rome, a neck wallet for my phone as well
--for women, a few scarves helped me "dress up" at night
I attended an evening concert on my own plus all of the churches, and I never felt too casual. My walking shoes were ugly, but they were black, so that helped with my overall look. However, I would have put comfort over fashion if I hadn't been able to get black shoes. I can't overemphasize how much you'll be on your feet, either walking or standing.