Hi Steve --
We've built a few such farm buildings on our similarly steep multi-angled sloped site using earth bags over a foundation layer of a few cement bags in place. No problems structurally -- however, be careful with surface water making sure to channel it away from the upper areas of the structure. We had a 16" cyclone hit soon after build and **thank goodness** we had first started with a French drain (a.k.a. mini swale) to channel water into a moat used for animal watering.... WHEW! SO -- poultry, swine, feed stores, and sprouting areas are in tact, have survived trials and tribulations, and remain in fine form (exactly as you mention!)
Good luck with your build!
Deo, P.E.
