I know you've already decided, but I'll add that I wouldn't bother. Our spare, under there a year now, still looks new without a cover. (Unfortunately I keep having reasons to pull it out, so I know what condition it's in.) It's protected from sunlight under there.
1995 Airstream Classic 30' Excella 1000
2014 Ram 2500 Crew Cab with Cummins 6.7L Diesel
Sold but not forgotten: 1991 Airstream B190
Sold: 2006 F-250 6.0L Powerstroke Supercab