Personally, I broke at least 4 or 5 turnbuckles, it was my fault for checking them more often. The final timne I did, I did a bit more investigation.
1. The more loose they get, the more prone to them breaking. With slack, the next big jolt will put more force on them than if they are snug. Once you break 2 or 3, you might put so much force on the remaining one(s) you too can watch you camper disappear in your mirror.
2. J hook bolts on the end of the turnbuckle break easier than eye bolt style ones.
3. The bolted down brackets the turnbuckle's attach to, are about 1/8-1/4 inch thick and any bolt you attach is round shaped steel bent to its J or Eye shape at the MFG. So you are putting the square edge of the bracket against and round shape of the J/eye bolt. Very little contact area to take massive weight/G force against each other when you hit a bump or accelerate. I found my solution right next to the turnbuckle's at the hardware store. Shackles look like a small horseshoe with a small threaded bolt between its ends. The best ones are much thicker where it will meet your bracket. You will need shorter turnbuckles and once you put these on and thread the shackles bolt through an eye bolt connected to the turnbuckle, tighten and lastly put a zip tie though one shackle and the turnbuckle to keep it from backing off. I recently checked my connections and it's probably been 18 months since I last put them on when I took my Fleet off (it's on 100%, drive daily on the worst highways at speed). I doubt they will need to be tightened unless I take it off again.