In MHO solid board mud guards act more like "scrapers" than guards and cause more damage than good. When we purchased our coach in 2004 we had Lazy Days install a ($100) Rock Solid shield on our coach. We have since towed more than 100k miles in every state(except Hawaii) and every Canadian province and have had zero damage to our "Toads".
In 2007 on a caravan to Alaska we traveled with friends with a higher end coach but similar size coach. We traveled the same roads at the same speed, day after day. On the worst roads their ($500) solid board "scraper" with "Zephyr" on it, dug up rocks that by days end covered the toad hood and roof with small rocks. Our Toad had only dust.