I would take my level reading from the refrigerator; it is the only thing that seems to care. When the frig is level, I would attach my levels outside by the tongue jack.
In my case, the frame, floor, sink, frig and slide all show slightly different level readings. Therefore, I lay a level on the frame, then I jack the trailer level and do a quick check of the frig make sure it is close.
I am sure there is a bit of frame torqueing when the slide it out or the ground in not really level and the wheels are on blocks.
Twenty six foot 2010 Dutchmen Lite pulled with a 2011 EcoBoost F-150 4x4.
Just right for Grandpa, Grandma and the dog.