Depending on what the soil is like, you may need to remove some of it (at least the organic material) and place some crushed gravel there. Or maybe some lengths of treated lumber in 2 layers, maybe 2-3' in both directions (90 degrees) to spread the load. If frost heaving happens there, maybe you need to go deeper into the ground. Soil can compress and expand season-season too and may want a means of periodically tightening support points (shims or jacks).
You'll definitely need to take weight off the wheels and have supports on at least one side of them. I'm guessing vertical supports every 8' or so but may depend on the frame strength. I would place the concrete blocks in a left/right direction to get a little better lateral restraint.
Not sure what your length and weight is. If you want to do a decent job of it, google what they do for mobile homes for ideas. They pour footing pads, use screw jacks and have stabilization rods/struts. Here is one example but may be a bit extreme for what you have/want.