Truck campers are meant to be used on OR off the truck.
Whoever gave you the idea that you HAVE to take the camper off the truck to use it, is mistaken.
If the truck sits level with the camper loaded, park the truck in a level spot, and use the camper exactly as you would if it were unloaded. There is no need to disconnect the truck from the camper, or lower the jacks.
That's how I always use my camper. I never take it off the truck while camping, never lower the jacks, never disconnect the tiedowns. I just use it.
I also carry some blocks to drive up on just in case I can't find a spot to park the truck and get the camper level.