Since you are travelling w/ someone in a wheelchair, you would be able to acquire a handicapped stateroom. These are very spacious. Plenty of storage and doors wide enough to accomodate a wheelchair. The bathroom is large as well. Ask your travel agent to check on the requirements for you. The price is the same as a regular ocean view.
I purchased the unlimited soda option on the last cruise for the first time. It was a 10-night sailing and I think I paid around $40. They affix a sticker to the key-card so all you have to do when asking for a drink is to show them your card. It was fountain only. Ask for a large glass as in the beginning they were giving me what looked like a juice glass full of ice. I maybe had three swallows before it was gone!
Our first RCL cruise was on the Sovereign. She is a beautiful ship.
We just got back from the Navigator -- there was plenty of storage space. All our bags fit easily under the beds and I kept the less needed things and dirty laundry in those bags.
The soda cards are $33 for a 7 night cruise for adults. At meals, you get large glasses and plenty of refils. We were given a travel mug with our cards that kept the drinks very cold. Since you can get unlimited refils, I don't think it matters so much on the size of the cup.