Unless you go for a custom made cover sloppy will likely be the norm.
We had the 21' to 25' model on our 2011 21SS. It would have been pretty loose, but like Drenjoey, we covered the spare tire, rear bumper, propane tanks and electric jack with the 'extra'. It worked out pretty well.
Our cover barely covered the camper top to bottom - leaving the tires almost completely exposed. If the cover you're looking at won't do much better you might want to consider getting something to cover your tires if they will be sitting out in the sun.
If your cover doesn't come with something to protect it from the gutter extensions (assuming you have them) cut off 4" sections of swim noodle and slip them over the extensions before you put the cover on.
2016 Roo 23IKSS / 2011 Tahoe
2011 Roo 21SS / 2011 Tahoe
2006 Bayside / 2006 4Runner
1999 SantaFe / 2001 Sienna
1967 Nimrod / 1998 Outback