Weight on the rear reduces the weight on the steer axle, which can lead to squirrelly handling if done to a sufficient degree. Some class C motorhomes (and probably some class A's) are rather prone that problem due to the very large overhang behind the back axle. Having more weight on the steer axle, at least within reason, generally has the opposite effect, making for a more stable vehicle and heavier steering.
It probably goes without saying, but take care that you don't block the lights, besides not blocking the airflow.
