I think the OP is asking, how do you level the camper first, by what criteria do you consider it level? If that is the case, the answer is simple; it's whatever YOU decide "level" actually is.
All campers are made different, and unfortunately, not all things in most campers are truly in-line with every things else.
Measure the level on your refrigerator and get it perfectly level, and the floor you walk on might feel like a roller coaster. Level the stove perfect and canned drinks may roll out of your refrigerator!
First, YOU have to determine what YOU want to be "level". For some, it's the refrigerator, for someone else, it's the floor. For another it's the shower floor. You get the idea.
My Springdale Keystone drove me absolutely nuts for several years because I finally figured the floor was built on an arch. So the floor, no matter how it sat, always felt like we were either climbing uphill, or rolling down hill, and never had to move the camper a bit! I had to figure out a way to reach a happy-medium, and then one day I realized a simply to determine what "level" actually was (at least for me) and I've been using this ever since.
The outside door frame! It's about as straight, up and down, side to side, top to bottom as you can get!
Using a simple carpenter's level, take your camper to a fairly level parking area, and using wedges, build up one side of the camper by rolling the wheels on the wedges (lumber), until the door frame shows level straight up and down for the side-to-side measurement. So put the carpenter's level on the outside frame of the door, until the bubble in the level is center.
Next, unhitch and using the trailer tongue jack, jack up or down until the same level shows straight up and down (front to back), using the same door frame, but now, putting the level on the inside of the frame. Once your bubble reaches the middle, you're level, front to back and side to side.
Now attach your outside RV level bubble!
If you don't attach an RV level bubble on the outside of your camper, you can always use the SAME simple door method. If the door does not swing forward or back when it's half open ... it's level. Works every time!
You will find, if you use the frame of the door to find your FIRST level, more than likely, the counter tops, the refrigerator, your shower floor, your bath tub drain, sink drains, and bed will be pretty level also. You cannot depend upon the floor, the trailer tongue, or the frame under the camper. Use the door. Works every time!
Good luck!
Edit: Actually, the "whisky level" is really a good way to level. It just depends upon where you put the glass. If the liquid in the glass is straight on all directions, and all sides equal (level), your good! The problem still is there? Where do you put the whisky glass? Well, that's simple ... you drink it! Then you don't need to worry about being "level" at all, especially after the 2nd one! (hee,hee!)
