I just have been playing with my Anderson a great deal trying to track down a vibration that set in above 55 mph....
To check the level of your trailer, measure from the ground to the belly band at the front and rear of the drivers side.
Then you can get an actual measure to the top of your coupler.
Choose the holes in the hitch head that puts the ball 1 1/2" higher and continue with your set up.
Use the belly band measurement to check your set up. You should be level when you are towing. I had to move the ball up one set of holes to get to level after towing home.
BTW - the vibration I was chasing was the lid of the storage box
. At speed it started flapping up and down and the whole bax started bouncing around. I tightened the latches and the vibration was gone. At least I didn't go buy a new truck to 'fix' it