Not trying to stir anything up...

I'm curious as to why Rose Marie is in a wheelchair. I know age is probably part of it, but I also know she had noticeable trouble walking in TDVDS Revisited episode from ~15 years ago. I wonder if she has had back or leg trouble or something else.

I didn't even know she used a wheelchair but like you said, age is most likely the biggest factor. The last time I saw her up and about was the 2004 DVD reunion episode. She's 93 now; it really wouldn't surprise me that she can't walk anymore. It's the sad thing about aging. But at least she's still active as far as giving interviews, etc.

IN a recent Twitter post she was using oxygen... Hoping she stays well.

To be fair to Rose Marie,

If one gives credence to her Twitter account, Mrs. Guy ALSO still makes a truly delicious homemade spaghetti sauce AND in recent days has been active in trying to help victims of Hurricane Harvey both human and four-legged (the latter via joining her fellow nonagenarian and onetime costar Doris Day's efforts).
IOW, she may not be as physically active at this stage of her life as she was in her youth but she IS trying to make the most of her abilities and trying to help those in desperate circumstances!