If the cruise excursion staff are organised then the first seats as you enter your coach should be for wheelchair passengers. When you are on board check with them if this is done and if not why not. Normally the wheelchairs are stored where your luggage would normally go.
Call RCI shore excursion dept and ask..01932 834325.
From family experience you will have to be able to get to the coach and walk up a couple of steps into to seats they do try and reserve the front seats but that is not guaranteed we found that the ships excursion staff were able to advise us on wheelchair friendly trips.Please be aware that not all ports have wheel chair friendly facilities,we found that RCI were very good with helping get you on and off the ship,but in some ports it may be better to stay on the ship,best of luck .