For what you're doing, I'd be more comfortable with the camper set closer to the ground. Especially if your repairs will require that all four jacks be removed at the same time. The X supports look like a good idea for stabilizing the camper at about the same height as when it's on the ruck. I think that was the inspiration behind their design; to reduce the amount of jacking required. I don't know as I'd be comfortable with using them as the sole support.
I'm working on my camper right now and have the rear jacks off. The camper is sitting on two sets of heavy duty jack stands, with a 4x4 laid across the top of each pair. The front jacks are down, but lightly loaded. The rear bumper on the camper is about 6" off the floor. The repairs I'm doing have required pushing, pulling, hammering, and crawling under the camper. Whatever you use, make sure it's stable enough for the work you need to do.
:):)