I bought a tyvek cover when I had a pop-up. I used it for several years before I gave up on it. The biggest problem I had was not being able to get the cover off late Winter when we wanted to go on a Winter Break trip down south. The dirt will still get under the cover and it will still get wet. So it will require a good washing to get it cleaned off after you remove the cover. The cover also wore down the trailer finish on some corners. I noticed it after the first year and padded those areas with foam rubber from then on. The cover does not last all that long though. When I gave up using it, it was getting pretty worn and had thin material that tore relatively easily. I had a lot of tape on it holding the tears together. For what it cost, I do not feel that I got any real benefit or advantage from it.
The RV roof and exterior is made to withstand the elements of Winter quite well. As long as yearly maintenance is kept up by checking/repairing any caulk/seals, there is really no good reason to use a cover. But, some people swear by the covers and if they want to spend the money for it and go through the extra work of putting it on and removing it, more power to them.