You can add nuts to any standard turnbuckle. Just buy a set of two for each corner from the hardware store.
The biggest thing though. When I had my ATC, I just stuck my hand through the little access panel every couple of days and snugged up the turnbuckles. Took a minute to do all four and I could make sure they were tight.
You guys are really overthinking this. You get in the camper every night to sleep. Reach in the little access panels and snug them up with a screwdriver as a lever every couple of days.
