Lock nuts on the turnbuckles are a necessity as an added safety item, perhaps WIRING each end so it can't turn on the body of the turnbuckle can't hurt either! However, if you are leaving on a trip, you would of course CHECK the alignment and the turnbuckles before you go, right? I would suggest that when you turn off the surfaced road or begin up/down any steep or tilting roadway, that you AGAIN check them along with any gear you have tied, bolted, or otherwise secured to your rig to insure it won't come loose or in the case of your camper, move around and incur damage. As a SYSTEM, each of the four turnbuckles carries a part of the load and as one or more begin to loosen, the strain is increased on the others which can lead to movement in the truck bed and damage to the camper or...begin to exert excess pull on the camper where the turnbuckles are attached.
A block in FRONT of the camper to transfer the forward loading of the camper to not just the front panel, but the floor of the camper keeps damage to a minimum. Additional wood blocking between the wheel well and the sides of the camper also help to stop sideways movement.
Its designed to prevent the "BB in a boxcar" effect wherein each time you go uphill or down hill or are bouncing around off-road you want to limit the possibility of any movement whatsoever.
I do think you need to check whatever method or securing your camper to your truck, be it turnbuckles inside the bed or outside, that its no different than checking to see if your saddle is still secured correctly to your horse! When you make a rest stop or however you need to once you learn if this is a problem for you on the road or going off-road!