This was my experience last year.
I've driven the MH for almost 13 year's without this happening including times on both the 99 and 5 in the same area your referring too but alas, last year while traveling the 5, I lost my water heater outside cover because of the rough road. I don't know exactly where it went bye bye but soon after Sacramento, I noticed it missing. May I suggest to those reading this to place a small "carabiner" clip or small padlock on the eyelet of that cover. I'm not the first to lose one.