I created the support posts by screwing together three sets of two 12’ 2x4’s and painting them matte black.  To hold the front two up I dug a 2’ deep post hole.  But rather than put the post directly in the ground, I took a 4” diameter PVC pipe and attached a cleanout adapter with plug, put that into the hole and leveled it vertically and with the ground.  That way I could leave the cap on, keeping debris from getting into the hole and also easily put the post into the ground and take it back out to store for the following year.  The dirt I dug up also served as topping for the freshly dug graves in the graveyard.  Two 2x4’s screwed together are actually too big to fit in the 4” PVC pipe and I had to cut off the corners so they would drop in.  For the post closest to the house I just attached it to a fence post with a couple of lag screws.

As far as getting all three wheels level on all three posts, I just eyed it up.  As long as they’re reasonably close they should be OK over that kind of distance.