Things built on top of the frame are typically not level, plumb, square or straight so I always use the frame and the bumper for leveling using a 3' level. If we end up having to be a little nose high or low (like when staying hooked up), that's okay but I don't like being off-level side-side. I have some of the larger stick-on levels but not sure if I'll ever use them. Used to have a round bubble level on the top of our electric tongue jack but wasn't very accurate.
Not all fridges have to be level - our Norcold fridge works up to 6 degrees out of level.
If you have a combo tub/shower and are not level, water can in some cases drain onto the floor. Our first TT did that and it had to be level front-back or higher at the front end.
