I added supports from bumper to rear jacks using bolted angle to bumper and fence clamps on the jacks.
Though I added to give side to side stability to jacks-tying them together thru bumper. The lower jack to camper mounts add virtually nothing for side to side-plus they are 2' from end of jacks.
This did firm up bumper flex a bit, as a side benefit. The bolted angle doesn't go all the way across, I didn't feel it needed to. 2 short pieces on either end.
If you wanted to go all they way across, Id use maybe aluminum angle to save the weight (& rust). The 'working' part is only ends stopping downward movement.
Other thought are rear stabilizers similar to our small jeep trailer. Used when unhitched. They fold down, then drop to ground. Mounting, height maybe problem-fine tuning, but simple. Just one example
http://www.etrailer.com/p-AT82297.html. Many styles of rear stabilizers.