The new Viking River Longboats I just returned from seeing are handicapped capable.
They have elevators and restrooms with automatic doors - in public areas.
I would not go in winter (when there are ice conditions). but a summer cruise should be fine.
As always, please verify this with the company before your book. But I am just reporting what I saw.
I am the editor, but I also speculate, ask questions and play devil's advocate. I reserve the right to change my mind.