At first I posted that the closest factory authorized Spartan service center was in Charlotte, Michigan but that's their main factory service center at their corporate headquarters.
If you use their service locator (...
go HERE), you can narrow down their authorized service centers. When I put in Indiana, I came up with the list that is attached below. The "MH" designation is for motorhomes.
If you don't need warranty work or something that you feel isn't necessary to use an actual "authorized" service center, there are a number of places you can have work done on a Spartan chassis. What exactly are you looking to have done?
.