I would never use any cinder block to support weight such a car or trailer. In high school we had a shop teacher do a demonstration. He placed a cinder block on the ground under a car, placed a hydraulic jack on top of the block and jacked the car up about 3 inches. He then tapped the cinder block with a very light weight hammer and the block shattered.
There is a reason that I have high end jack stands in my garage and will not ever use concrete blocks.
I will not apply for a Darwin Award.