Entrance door jambs don't crack by themselves. Never heard of one cracking, unless, the trailer was being towed. With that in mind, lack of proper running gear balance is the culprit. In this case twice should be two too many. But unless the balancing is done, that crack will happen again and again. Adding "L" brackets to it is a waste of time and money, as it will break as well. I would also bet that the hinges on the entrance door are not in good shape either. More clues to the cause of the problem would be missing or loose rivets inside as well as outside. Check the striker bolt on the entrance door lock. If it has a groove cut in it, that's another clue of vibration. If you do find that groove, replace the striker bolt. If not, someday you will not be able to get out of the trailer, except through a window. Check the striker plate and the small brass shim that's on the inside of the striker pocket. You will probably find it worn as well.
And finally, check the axles. If the rubber rods have deteriorated, bottoming out will occur when traveling. That will punish the frame, shell, furniture, as well as the door jamb.
To learn how to check out your axles, go to our web site, inlandrv.com Click on articles and then click on "Dura-torque axles."