The footers are great and they do give additional support.....for the building. The footer is only on the perimeter and does nothing to support the weight of your coach. I would go with a 6" slab, reinforced with 6x6 mesh and using 3000 pound fibermesh concrete.
By the time they cut expansion cuts in the slab, you will need the additional thickness to prevent settling of the slab.
I would also specify that the slab is poured LEVEL, rather than having a slope towards the door. Even here in Florida, where we get a lot of rain, the level slab is much easier/safer when dealing with vehicles that will be worked on and on jack stands. Before the garage is built, a level slab will allow any water to run off just fine.