Travel Trailer Cover? Good/Bad Idea?

We have used covers in the past, but they didn't seem to last more than 3 years and they let the water and snow go through the cover leaving the roof very dirty. So for the past 6 years,we have used a 30 x 20 ft tarp. It works better than any cover we ever had and it doesn't leave dirt trapped on the roof like the covers did. We don't hold it down using the grommets. We use 2" strapping we bought at the local hardware store. You can cut it to length and they have latches to connect the 2 ends together.We use the straps over every year. Then we use ropes through the end grommets and tie them together to keep the wind from getting under the tarp. The tarps cost around $80 and usually only last one year. But the covers were over $300 and only lasted 3 years so we come out ahead with the tarps and the trailer stays cleaner than with the covers.