In the few instances when we have been assigned a site that is way off level, we head back to the office and ask for a more level site. On the way back, we scope out empty sites so that we can ask for a site that we know is better. We do most of our traveling during the Spring and Fall when kids are in school, so we seldom run into full campgrounds.
Also, it isn't necessary to be completely level to protect the refrigerator and be comfortable living in the RV. If a site is within 3 inches of level, we don't bother with blocking up the wheels.