So the project begins

I dont want to offend anyone so Ill tread lightly but I'm not putting my 250lbs in there walking around on saw horses. I wouldn't even trust the saw horses with it just in storage

No offense taken, but the sawhorse I custom built have a designed load strength of over 4,000#. That is just the standard heavy duty design you see commonly used under campers. If I put you blind folded in my camper versus yours, you couldn't tell me which one was on sawhorse at truck bed height and the one that was on the concrete blocks. I would not try it on store bought sawhorses though. Plus, I will give your blocks the edge on strength because you do have them turned the right way.

I would mount the two jacks on the drivers side in the exact same position. Just drill the new jacks with the needed bolt pattern. The existing jacks will come out easier than you think because odds are it is only the heads that are so badly rusted. Hit them with some PB Blaster, let soak over night and then pop them loose. Reason for the same location is there should be some extra re-enforcement in the jack locations.

The one on the passenger side you will probably find mounted with carriage bolts under the seat cushions.

I would carefully check the wood in those locations as it seems to have some rot. Say this because of the metal is bowed at the jack points. However, that just may be poor design. Good thing is with the metal siding, easy to roll up the bottom side to repair.

The reason it got wiggly the last two inches is because that is where it lost all contact with the truck bed. As long as it has even a little weight in the bed, it greatly steadies up the camper. Except for not being able to use it when on the jacks, the three legs isn't a problem and almost no less stable than a 4 jack camper during the jacking process. This is because on a 4 jack system with manual jacks, unless you have two people, one of the rear jack almost never has enough weight to provide additional stability. Most of the time, one of the rear jacks will be completely off the ground. Unless of course you are only taking 2 or 3 turns per jack as you go around.

I grew up with a 3 jack, 11' Franklin which we never had problems with. I now have a 4 jack, 8'6" Lance. Both were manual screws jacks. My 4 jack camper wobbles just as much as the 3 jack camper.