Keeping your RV covered is a big help in preventing deterioration from UV rays, not be mention keeping other weather elements off of it - ice, rain, show, etc. We went through several off-the-shelf ADCO coveres before we wised up and built an RV Port. None of the stock covers fit a class B appropriately - as was already mentioned, you'll have to get creative in dealing with the significant amount of extra material present for what would normally cover the overhead cab area of a class C.
If you're going to spend over $1k for a custom made cover that will last, at best, several years, why not put that money toward a permanent structure. It is, without question, the best investment we have made with regard to protecting our RV. I realize this is subject to codes in your neighborhood, space available, and other issues. But if you can, I highly recommend it. Our RV port cost around $5.5k and stands 13 ft tall, 14 ft. wide, and 35 feet long. The height leaves room for me to access the roof of the RV for maintenance, cleaning, etc. You've spent something in the range of 100k on an RV - spending another $5-7.5k to keep it out of the elements was an easy decision for us.
With the RV Port, we can keep the unit plugged to shore power and have the AC running, and fridge/freezer cooling down while preparing for a trip (something you can't do w/ a cover on the unit). You can also do maintenance on the unit while it's protected from the elements. Another nice thing - it stays much cleaner than with a fabric cover. Lastly - no lugging a heavy cover up to the roof and unfolding it / teasing it down the sides / securing it w/ cords and rope. I found that I would often leave the unit uncovered for a few weeks if we were about to take another trip soon instead of covering it, because of the hassle of installing the cover.
And if/when you sell your home, a prospective buyer will no doubt find an RV Port an attractive feature under which to store whatever toy they might have.
