Regardless which state you are in, you should always inquire with the state requirements as well as get familiar with the base national HUD guidelines. I have installers licenses in multiple states and the requirements from state to state will vary.
Please take that into account when you get feedback. Erik Hanson is correct - Amy Bliss is the best resource in WI. In fact, the WI MHA is the best association IMO.
Regarding WI, there are different requirements for pre and post 2007 homes. Pre 2007 basically defaults to base HUD guidelines - level lot, no vegetation, soil strength requirements, etc. This is very simple. In this case it can be exposed to dirt, however, its always wise to put a vapor barrier underneath.
Post 2007 is where it becomes more difficult as you will need to deal with frostlines, etc. The types of WI state approved sets are concrete slab, concrete piers, frost free or frost protected shallow foundations (some caveats) or HUD approved helical piles.
Also, its best to check with the home manufacturers DAPIA for approved systems.
Hope that helps.