What Is A Level Trailer?

The “level” exercise as you note is a learning experience and for first time newbie trailer owners can be a mystery as there is a method to the madness if you will.  

Before unhooking the key is to check for “level” side to side on the trailer and use leveling pads under one side of the trailer as needed.  Once the side to side level is attained I can place my wheel chocks and unhook from the truck, then adjust the tongue jack to attain level from front to rear of the trailer.  That in a nutshell is it....  For those of us with showers it impacts water flow as it drains and may be the difference in water collecting on one side or properly draining into the gray tank.  And not to forget sleeping comfortably and the other issues you noted.

There is definitely a process to learn here and a method to the madness in attaining “level.”  But once you unhook, realize you messed up (trailer is leaning honey!), re-hook up and strain that side to side level, etc. then things finally sink in, you get it and move on to the next chore. 🤣👍🏻

I use a small level too, but also have the adhesive stick on levels on the front of the trailer.  One is inside the tub at the front and the other on the side of the front of the trailer frame.  Makes this task fast and easy.  👍🏻

@GatorEgg, very good point re what does "Ievel" actually mean?

I initially checked the level of my T@B 320S (side-to-side and front-to-rear) using a carpenter's level on the inside floor and a smaller level at other locations to double-check the level status (e.g., inside the refrigerator [most critical], kitchen counter, front frame, etc..)  Once I was satisfied, I mounted small bubble gauges on the driver's side of my Boondock frame so I can see the settings while unhitching.  I followed up by checking the level visually by parking on what appeared to be a very level parking lot.  All looked good and the levels make it very easy to check the camper's level status while unhitching.

I agree with @Michigan_Mike's comment about leveling side-to-side first, using some form of leveling block(s) or other type leveler on the low side wheel to raise the camper.  Then, unhitch and level front-to-rear by raising/lowering the front jack stand as needed.  Also, I agree that if you are going to be using the air conditioner, it is a good idea to slightly lower the front end for proper drainage of the A/C drip pan.  (NOTE: in the photo below, my 320S was still connected to my truck, but leveled side-to-side, so the front-to-back level is up a couple inches.  Once unhitched,  it is very easy to crank down the front end of the camper to get it to an (albeit unscientifically achieved) level position.) 

  

We are using this electronic level sensor via bluetooth with android app to tell us how to adjust left-right and front-rear axes (see page 2 here)..
https://tab-rv.vanillacommunity.com/discussion/comment/109938#Comment_109938

Because I use a power mover and a BAL Leveler, I unhook first in the front of the campsite, drive the camper into an exact spot (with two fingers), then back the truck in out of the road. Then level front to back, then put the BAL on the low side and crank it up. I have a 12V fridge so not worried about level too bad there, but initially set  level on the CoolCat so that it drains properly, then put stick on bubble levels on the top of the wheel wells, and on the front handles.