The various trailer / rv covers are on sale at Camping World....but I wouldn't waste $400 on one to cover a 21' trailer again. They are very heavy, awkward to put on / off the trailer even if you have a roof ladder and twice as bad if you have to use stepladders, easy to tear, snag on everything imaginable, take up huge space when they are "folded up" when they're not on the unit and only last a couple years.
I have also used tarps, and find that the wife and I can use a couple of poles or paint roller extension handles and 'walk' a tarp over our trailer in a couple minutes and secure it in place with bungee cords and / or paracord (small rope). They last a couple years, fold up pretty small (compared to a cover), cost 10% or less than a cover for the size trailer we needed, ventilate well enough to not cause mold, and if they get a torn spot, I'm not cryin' over $400+ dollars that now has a hole in it.
To get the size tarp you need, measure how tall the body of the unit is at it's maximum (the AC covers) multiply times 2 (for both ends) then add the length of the unit, for the "long" side. Then again twice the height plus the width of the unit for the "short" sides and find a tarp that at least covers the "long" way. Our unit, being 22 feet long, was covered well by a tarp 26 feet on the long side.
