If You have power jacks, I would do it the same way you build a house foundation, (I have built three houses from the ground up, a small barn, and my travel trailer barn)
This is all based on a permanent site.
Get your fifth wheeler where you want it, and then using power jacks get it higher than you want it.
Next figure where you are putting your supports, ( I would use concrete blocks)
Dig down a few inches for each and after you make sure you are on a solid surface put a block in each hole.
If it was me, and the RV was going to be there for ever, I would dig down a foot or so, and make the hole twice the size the perimeter of each block, and then pour a three or four inch slab of concrete, prior to putting the block on top.
Don't worry about the top of the bottom layer of blocks being level to each other, you do that as a final step.
Then determine how high you want the foundation.
When you determine the height, pick one pier (call it pier "A") as your height, and then you can either use a tube water level, or a builders transit, (you can rent them) and from pier A, figure the level of each of the others in turn. Use treated lumber (2"x 8"s and/ or 1" x 8"s in combination with shims. Cut the wood the same size as the top of the blocks for aesthetics.
Once you have all the blocks the same height, just lower the RV onto them.
Jack L