"I don't see how that board with 2x4 blocks without a strap would keep a tank in place on any kind of bumpy road. bumpy "
As far as I know, the tank never moved. Of course it had no where to go either. It was somewhat supported by the green tube in front and the wall in back. If it did came out of the cradle, it always rolled back into the same spot.
Like I said, we no longer have this trailer (or the propane tank either), and I'm a little older now, and a little wiser. I wouldn't do it this way again if I had to do it over. I'd keep it upright.