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

The factory jack should be rated for a fully loaded truck (GVWR) plus a safety margin. Aside from a camper, you might have other cargo that's not able to be unloaded roadside to repair a flat tire, maybe gravel, mulch, bricks, whatever. Also remember that the jack isn't lifting the whole loaded truck in the air so you can work on it, it's only lifting one corner a couple inches so you can change a tire.