Side to side, I look for objects in the back ground that are straight up and down, like the edge of a building. I then try to eyeball the side of the camper with that straight edge. After a while, You just get a good feel for what's level. A simple carpenters level across the tongue is another alternative.
Level front to back, just open the door. If it swings forward, the front is too low. If it swings back, the back is too low. Once the door no longer swings, it's level.
Forget about attempting to level based on the floor. They are not always flat. My last camper had a slightly bowed arched floor, that drove me crazy. I learned to level by using the door swinging. The outside door frame should always be straight and perpendicular to camper. If you get the door frame straight up and down (front-back) and if you get it straight up (side to side), then that's the best you can do. Forget leveling on the floor.
About those sensors on your tanks. They are a waste of effort. Ignore them and learn by practical experience and just keeping a close watch on your usage.