Whichever direction is easier should be fine, broadly speaking.
I tend to prefer to have my class C tilt towards the back for long-term storage rather than towards the front simply because I'd rather have less water collecting around the cabover section in case a leak should develop there. The cabovers have a lot of seams and such that make them a common site of leaks--at least the ones built like mine is do.
It is also very nice if you can arrange things so the water from the roof doesn't all descend right in front of the entry door!
