Not knowing any better when I started, I leveled my cab at the door window openings. Laid the level on the window openings (minus glass and trim) fore and aft and across. Right or wrong, that's where she sits.
Garage floors are NOT level. They're poured to drain out. They'll be high at the inside wall and low at the garage door. This slant is purposeful; snow, ice, and rain water on your car will hit the floor and drain naturally towards the garage door. Anything else and you'll have an indoor lake.