Jamie,

6" of #3,500+ psi concrete on your sand base should be more than adequate. This gives you over 20,000# per sq in. You will want to make sure they use screen and fiber reinforcement. This will take care of any uneven compactness in the soil, which shouldn't really be a big deal over sand. Although screen reinforcement seems to be on its way out in favor of fiber, you can use both. Also, make sure you ask for commercial fiber, which really just means twice as much. Why some don't prefer the use commercial fiber is because it can be seen after the concrete is finished. The fine fiberglass strands often are visible on the surface. I don't consider that a big deal. I've done it twice. My last garage I used a two part epoxy with paint flecks and it looked great.

Two keys to eliminating or at least reducing cracks, that are almost inevitable, is to make sure the concrete has been given ample time to cure, that it has enough stress joints, and that the ground has been compacted after leveled. My current pad, built on soil likely similar to yours has been crack-free. It's a minimum of 6" thick with shovel width and 12" deep perimeter. The perimeter has one piece of rebar. I did not use reinforcing mesh on the advice from several in the concrete business that it's not necessary when using fiber. I used commercial fiber. Home builders in Florida try to wait at least a month before pouring after the soil has been prepared. I can't say I've seen the same for garage builds. In an ideal world, they would also use a heavy roller on the prepared soil. I've never seen this on residential builds here. What is typical here is for the grader (tractor) to be run back and forth over the ground.

Your coach's floor load is less than 3,000# per sq in when it's on it's tires. Depending on the floor jacks that are used this number can be significantly greater. That's why floor jacks should have weight distributing pads.

Where you live makes a difference. The build process here is nowhere the same as what I saw in Virginia.