Water management and shed base for two sheds built against the houses on adjacent properties
Sloping the soil would never be a bad idea, but more important is supporting the perimeter with a proper foundation. Why do you need a slab at all? Assuming you have a reason for using a slab I would normally recommend a "floating slab" setup, but that is probably not feasible because you are placing a pre-built structure there. A floating slab would require the perimeter foundation to extend above grade in order for your floor to be level with grade. Not possible because your walls and floor are already built. An alternative would be one like shown below (dimensions shown could be reduced appropriate to your structure size). It is critical that the soil and gravel below be compacted.