Don't forget that the rim has to match what your Sunseeker has also (lug pattern, center bore, offset). Also, aren't the rear wheels dualies? Having different sized tires would probably do a number on your axle. And I don't know enough about the setup, but are all six rims even the same size?
What is the provision for the Sunseeker for a spare anyway? It doesn't come with one?
Current: 2013 Flagstaff MicroLite 21DS
Past: 2000 Fleetwood Mallard 19N
TV: 2013 Nissan Pathfinder SL 4x4