g,
While I can't speak to a cover for a trailer, I will tell you that folks who use a cloth / fabric / whatever cover for their motorhomes and/or travel trailers are generally very unhappy with the results. The wind wears on the paint and they are a real pain to install and remove, especially when wet from rain or dew.
If you are truly unable to keep your trailer in a stall of some sort, you're doing about all you can by keeping it washed and waxed with a good quality product. And for goodness sakes, keep the tires covered and out of the sun and coated with 303.
https://www.napaonline.com/en/p/NCB3...ng_dcoe_google