I know what the problem is. when he is unhitching the front of the truck is up and the rear is down along with the trailer making it only able to put a 6" piece of wood under the tongue. However looking at the picture it doesn't look bad. if it is here is what I would do. before you unhook chock the wheels then raise the tongue and lower the front jacks. then raise the tongue jack and add more blocks to be able to raise the trailer to level. the jacks will be able to support that weight and will not damage the trailer because the rear jacks will not be down. damage comes from when you try and lift the trailer with both front and rear stabilizing jacks, they will twist the frame when used like that. stab jacks at the front corners used the way I described will do no damage.