There are designated area to store ECVs and wheelchairs for those who don't need them all the time. I think on the Fantasy it is near the midship elevators but I don't recall which deck. On the classic ships I think the storage area is by the aft elevators but again I don't recall which deck. Guest Services can tell you the exact location and I imagine the officers who greet you as you board could advise you as well.
If you will not be using the wheelchair on board or using it infrequently, I think it would be kind to NOT book an accessible cabin as there are not many of them and they can be very hard to book for those who need a wheelchair all the time.