1st and biggest question is what does the local codes allow. Many areas don't want you living in an RV.
Why do you "need" tie downs? If it's mobile, hook up and get out if a hurricane is coming.
- If you want them anyway, get some earth screws and put them in prior to casting the slab, so the tops stick out. At that point, it should be pretty simple to connect them to the frame (worst case a welder could attach some rings).
What's the weight of the unit? For most, 6" should be enough. Make sure to put a slight cross slope on it and have it set a few inches higher than the surrounding ground so it doesn't sit in a pool of water every time it rains. Any driveway guy should be able to do it if you don't want to do it yourself.