Looking for idea on how to support my camper of truck?

If you make your X pattern only 12 inches high, then you put two at each end instead of just one. This would then span the whole width of the base of the camper. Lay four two by sixes flat onto the X pattern, and set the camper down with the camper side edges supported by the two by sixes.

This holds the camper off the ground at 13.5 inches up.