Does it have a TYVEK (white perforated looking material) top on the cover? Mine does and all of them I have had have been so equipped. The tops of mine have shed water very well, the side will get soaked but does shed the water away from the coach. I have never had a pool of water ever. I do not cinch mine down as tight as you describe. I have had mine come off or rip in the wind.
From your description it seems plausible this contributed to the problem but there really is no way to be sure is there?
My covers last three to five years. I favor covering mine but most people do not cover or even like the idea judging from comments in the past. I believe mine have extended the life of the exterior of the MH.
I do all my own stunts
03 Dolphin LX 6355, Workhorse W22, 8.1 vortec, 04 CR-V, Blue Ox, Brake Pro----Norm, Barb and
Doc(He's a PhD)