Any additions I have done were with 1/8" stainless steel plate to get the necessary strength. The load on one jack can be over 1,000 lbs. and so this needs to be factored into the design. The wood construction is also marginal and adding a metal corner piece so the weight is supported from below would help a great deal, so long as the metal itself does not flex. With steel the material can be half as thick as would be needed with aluminum to provide the same resistance to flexing.
To avoid downward stress on the camper's anchor points keep the turnbuckle loading to under 300 lbs. per corner. With Fastguns for example an extra 1/4" of compression can result in 600 lbs. of force pulling down at the anchor plate and damage the camper.
Nice job on the camper rebuild. Good time to also replace plumbing lines and connectors as well as these are not done at the factory in a way to allow easy replacement in the future. Also a good time to add one of the aftermarket tank level sensors that fasten to the outside of the holding tanks and provide accurate fluid level readings.
