If I turn too sharp, my trailer will hit the cab. Am I doing something wrong?

We should be able to get you enough cab clearance for your setup to avoid hitting the cab of your truck. Please make sure you are optimizing the offset that the hitch can provide, the hitch should be pinned in the rails with the ball closest to the tail gate (hitch can be rotated either way but in shorter bed trucks have the ball closest to the tailgate). Now look at the coupler block that is on the king pin of the RV and the ball socket should be towards the front of the RV in front of the king pin (like the base the block can be rotated 180 degrees to increase or decrease offset and you need the maximum offset available).

Double check those and if that is already being done then if you could send a picture of the setup connected maybe we can think of something else.