Hello Jean - Glad to have you with us!
Yes, you can get "custom made" covers for your HiLo. I had one made for mine, out of Sunbrella fabric and I've been quite happy with it. It was about $600, which included shipping. Here's a link to the thread where I reported on it: https://www.hilotrailerforum.com/f32...er-cover-5677/
Don't use a plastic tarp. You want the cover to "breath" so that it doesn't trap water under there, against the trailer. The cover is there for protection from the sun.
Buy covers for your tires too - they degrade under sunlight if the trailer is stored outside.
You certainly CAN wax the sides of the trailer, although I've never done that. I just wash mine now and then. I think I'd use automotive detergent instead of laundry detergent though. I don't see how this would hurt a rubber roof. A rubber roof should be obvious - mine's aluminum and has a "pebble-grained" feel to it. I think you can tell the difference by touch.