A friend & I changed a flat on a V-8 Vega with no jack on the side of I-10 somewhere East of Desert Center at 3AM. People were moving and lifting heavy, awkward things long before camper jacks were around. The basic jist will be to support the camper while the rig is higher than normal off the ground, and then sag the truck out from under, OR support the camper with the rig at normal ride height and then lower the truck out from under it either by digging or possibly by simply lowering the tire pressure.
Big question is, do you need access to the camper while it is off the truck?