F350 where do I jack from? / would this plan work?

Yup under the axle tube or differential.
If truck is empty I’ll jack it up under a shock mount or a control arm mount, but not with a load.
6000 lbs is a pretty cheesy Jack for the back of a fully loaded truck/camper. Although your camper is lighter than some, I’d ditch the contraption for a simple 10 ton bottle jack and a few squares of 3/4” or thicker plywood which double as camper jack pads and leveling pads to level up the truck when camping.