Phil, you bring up a good point that not all truck campers are built the same.
It’s always a good idea to check with the manufacturer. Northwood told me years ago their Arctic Fox freshwater tanks are supported within the frame to allow for the TC to be lifted with a full load of water. They also said the Happijacs were strong enough to allow for the TC to be supported while camping without the need for underneath support.
Lance does have a video on their website showing two families using their rigs off the vehicles with only jac support.