I actually put a torpedo level on the kitchen slide trim about mid-coach for front/back and for side to side, I use the trim above the pantry cabinet that is left/right.
As others have said, on soft sites, some type of jack pads are helpful. Heck, mine are 12" x 24", and I have sunk them in the ground on very soft sites! If the site is VERY unlevel, consider additional blocks under one or two of the jacks to get more lift for the jack travel.
Also as said, lower the front/center jack first and get the front/back level or front high. Then lower the rear jack on the side that needs to come up the most. You should be able to level with just these two, then lower the third jack just to stabilize.
At least, that is my process and seems to have worked over the last 9 years.