I've been on 4 RS tours now as a single woman. My husband joined me for a 5th one.
It's been my observation for all of them that the tours dominated by couples, and especially when more than one couple are traveling together, are exactly as you described. Some couples seem to be joined at the hip and vanish into thin air within seconds of a group activity being over.
It's been my experience that the more single women on any tour, the more likely all the tour members are open to inviting me to join them or to welcome me if I ask to join them. Please note that on all 5 tours, there were no single men traveling totally on their own, even if they did not share a room with their travel partner.
In 2nd place for friendliness and openness were the tours with families with 2-4 members and including at least one older or adult child. Examples include mom + daughter or son, mom + daughter in one room and mom's sister in another, mom + son in one room and dad/grandfather in a separate room -- the possible combinations are endless.
Unfortunately, you don't have a clue as to the demographics of the tour group until it's way too late to cancel, so you may be lucky or not with that.
As for dinners alone, this has never been a problem for me, but if it is for you the planning ahead advice previously given is good. I find that there is so much food on these tours and they are so active that I rarely feel like eating dinner at all. I'd rather have a bite in my room. Dinner out is fun, but it inevitably means not getting back to the room until late and not having much time to sleep before getting up early the next day.
The best part about RS tours for me is that every one I've been on has included unique experiences that I would never have even thought of or been able to do on my own.
The worst part that affects my desire for joining people for dinner is that I don't drink. I have never seen anyone get sloppy drunk on any RS tour, but on all 4 of the ones I've been solo, I've also been the only person who did not drink except for the kids who were below drinking age for the country we were in. Some jokes are just not as funny to a sober person as they are to someone who has 2 or more glasses of wine under their belt before dinner is even served.