When I built my camper, my friend and I worked on this pretty extensively. Needed to be strong and durable. I managed to pick up a decent set of steel camper corner attachments for the legs long with a rebuilt set of hydraulic legs for a fair price. The corner brackets are attached into the steel corners. I drilled and tapped 5 bolts directly into the steel uprights. Also, the steel in each corner is reinforced with a square plate to assist in rigidity and overall strength for each corner.
I can lift the camper with one corner and it hardly racks at all. Corner Jack security was top of mind for all the reasons you see out there and all the horror stories of corners failing.
1x1" steel corners - thick wall tubing plus a 1x1' steel plate
Ties all the corners in together (squares because easy to cut)
Brackets are pretty standard used old camper brackets. Worked out great, but extra holes I didn't use. Super solidly attached and bedded in with a bunch of Sika.
Lifting this thing still freaks me out every time. lol