Chandra, we took the Great Rivers of Europe in a large family group.Three of our family members had serious mobility issues. One had congestive heart failure and couldn't walk far or do many stairs, one had major knee issues and couldn't walk far without pain, and one had had a stroke, so balance and steps were both problems. They all used canes of various types. Because we had many able bodied family members to help and were willing to go to extra expense, we were able to accomplish this trip and have everyone enjoy it.
On GCT river cruises, you have three options when the ship docks.. You can stay on the ship and watch the world beside the docks. (Sometimes it's interesting, other times it's not, but it's always better than sitting at home watching TV! ) You can also take the excellent regular walking tours that are offered and see the city or village you are in....and then go out on your own in the afternoon. This is, of course, what most people do. Or, you can do your own thing, which is what members of our group did. You can see quite a lot from the ship itself, of course, as it sails down the river. And many fun things come right to the ship for you to enjoy there.....glassblowers, folk dancers, strudel-makers, etc.
When the ship would dock in a town or city, those of us who were younger and stronger took turns going out to sightsee with the tour group or on our own. But we had the program director arrange for a taxi or a van and driver in each port. so that those with health problems could still see some of the area we were in. They only had to walk down the gangplank and get into the van. Three of the more able bodied would take turns to accompany the three with problems, helping them in and out of the van, staying with them, trouble shooting any stairs or paths, accompanying them to whatever sights they wanted or were able to see. In one city, for instance, the van took them right to the door of the Gutenberg Museum, which they could navigate themselves because it wasn't too large and also because it had good elevators. Then the van (which waited outside) took them right to the door of the cathedral so they could see the Chagall windows. Then the van took them back to the ship. (The rest of us took the walking tour of the old town, visited the museum, walked through the marketplace and up the hill to the cathedral, walked back down and had lunch, took a little tourist tram around the city, and walked back to the ship.) The next day, those who helped the mobility. impaired would have the "day off" and someone else would rotate to being a helper.
Our program director was able to generously provide time to tell us which sights were accessible for everyone, and which were not. (Some cathedrals, for instance, have access only by stairs. Some monasteries were simply too large.) Sometimes the van took a "driving tour" of a city, and in a few places the van passengers went out of town to other smaller villages. They might eat a leisurely meal in a sidewalk cafe, and take a look at a different area. Sometimes the optional tour was by bus, and they could ride along on a regular tour and see the sights from the window, and then enjoy waiting in a smaller square or area of the town being visited until the return trip. In Amsterdam, they really enjoyed the canal boat tour.
There was at least one day when each of our mobility challenged people stayed on the ship, tired. But they were so thrilled to be out seeing the world, even with a limited itinerary, that we all agreed that it certainly was worth the effort of planning and executing the adaptations we chose. They loved the trip and have talked about it often. If you need the kind of help I have described, perhaps you have a younger couple of relatives or friends who would be willing to travel as your helper/companion in exchange for a trip to Europe.
As others have said, your scooter is probably not suitable for these small ships. But, because they ARE small ships, perhaps your walking abilities would be enough to enable you to get around on them more easily than you can on a large ship. If you can go up one flight of stairs, and half an hour later, go down, that would be suitable. Only one deck of our ship, the Concerto, was not accessible by the elevator. And the stairs to the top on that trip actually had an individual "seat" that went up and down the stairs, but I don't know if every ship, every trip, has that available. Check out the length of the GCT ships and see if they are too long for you to walk, at your own pace. You could also arrange to have a room right by the elevator, to lessen the distances for you.
If the kind of trip I have described isn't possible for you, then go for the big cruise ships where you can take an electric wheelchair or a scooter. Whichever type of trip you choose, you have to realize that because of the cobblestones and stairs of the European landscape, it will not be possible to see every sight along your route. But with some careful planning and determination, you can still have a wonderful, interesting trip.