Going down the road, there is enough bouncing, that the fridge won't be an issue as the fluid will still be able to flow. It's when it's stationary level becomes more critical...but even then 1/4 bubble shouldn't be a problem.
It's unclear from your comments but there are two adjustments you can make to the hitch setup:
- Set the hitch to the lowest set of holes.
- Set the trailer pin box to the highest set of holes.
- The only catch is you still want to maintain at least 6" of clearance so as you go over bumps and dips, the trailer overhang doesn't hit the truck bed.
Not sure what options you have to raise the trailer if it's not the typical leaf spring style.