I had one TT in the past, that no matter where I put a carpenter's level, it was always something different. Even the floor.
I finally figured it out the floor was actually arched, low on both ends, high in the middle. No matter where you walked, we always felt like we were walking on a hill.
Trying to find a spot that was true left-right, and end-to-end level was a real challenge, until I realized one day that a few things in the camper (and ever camper) are always "straight". Notice I said "straight", not necessarily "level". The walls and the door frame is always "straight" and parallel or perpendicular to the trailer frame.
I also realized when the trailer was "straight" the outside door, and the bathroom door, and the cabinet doors, and the refrigerator doors did not swing open or closed. If the trailer is "straight" the doors should not swing open or closed, but stay wherever you put them.
At that point, I realized the outside door frame was "straight" to the frame also, straight to the roof, and straight to everything else. So I used the carpenter's square on the door frame, up and down to so the side-to-side direction was "perfect". And it worked! Then with the tongue jack, turned the carpenter's level the other direction (up and down) until the small perpendicular bubble was centered. Worked every time after that. No doors ever swung again. No cabinet doors swung open or closed. And the refrigerator door did not swing. It stayed where I put it.
After that, I was able to attach a small bubble on the electric tongue jack.
Next trailer I tried something different. The outside door method worked just as well, but I wanted something that I could use while backing into a site. Being a non-professional (paid) carpenter, I used my carpentry skills and came up with a rather ingenious way to indicate if the trailer was level, or at least close enough, the stablizer jacks would let me make minor adjustments.
Here is what I did.
I used the principle of gravity and the plumb bob. I put a piece of tape on the front cap of the trailer, a piece of tape on the inside of the rear truck window, and hung a hanging stick near the rear of the truck (since I had shell).
When the stick, which swung by gravity, was parallel to the vertical tape on the trailer, the trailer was level side-to-side. And this worked very, very well. The only draw back, I had to give the stick a bit of time to quit swinging. I always followed up using the carpenter's square at the door, and I was always dead on!
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