I used Happijac spring loaded up front and standard Happijac turnbuckles on the rear with an 88 F250 and an old 10' or 11' Lance. If you follow the instructions for installation they even kept that land-barge in place. They mount outside the bed so you can check them frequently. I found they did not loosen, but on washboard roads the eyebolts in the Lance had started to bend a bit. That allowed more movement and more damage but the nuts on the threaded shafts were still TIGHT which indicated they did not loosen!
The concept of a spring loaded turnbuckle is correct for a camper & truck bed that have some flex to them. I think using some wedges between camper and wheelwells is an excellent idea as you can stop sliding/twisting on the east-west axis that way.
Taking a walk-around each time you fuel or stop for eats should include checking on turnbuckles no matter what kind you use. Those in the bed of the truck may be a PITA to check, but it should be a regular part of your pre-departure checklist and at least something you check again prior to packing up for the return....
If yours do loosen up you will soon learn how long that takes and be prepared to retighten them accordingly until you find a better system than the one you have or a better way to tighten each turnbuckle up.
You only need to see ONE slide in camper that has slid out on the freeway to remind you that wind resistance, vibration and failure of the system to hold it secure can ruin your trip....