Hi
I currently use a heavy duty silver 20' x 40' square tarp that I purchased at Tractor Supply Store about 5 years ago. This is the second trailer I have used it on. I paid $130 for it. Prior to that I paid $300 for an ADCO custom fit cover for my former 31' TT. The cover lasted one winter, with tears at both rear corners and at the antenna location on the roof. The tarp has no tears after 5 years.
While the tarp is not custom fit, all it takes is a little creativity when tying it down to have a nice tight fit. I use about 150' of 3/16" rope (cost $7) to tie it down. It is as tight as a drum and stands up well to the winter winds here in Western NY. I never experienced moisture or mold problems, as I have vent covers and leave the roof vents open. My rig sits under trees and the tarp also stays very clean even when covered with wet leaves. I cant say this for a tyvek material fabric cover. Also, the plastic grommets on the tarp do not scratch the trailer sides, keeping in mine it is tied down very tightly. It takes me about 2 hours to cover it.
I would not cover it any other way.
Just my 2 cents