This question is asked many times. My suggestion is the outside door frame. Get it level front to back and side to side. Easy to do, just set a carpenters level on the hook side of the door (not the hinged side). Set it right up against the catch that keeps the door from pushing backwards. The level will be in the vertical (up and down) position. Get it level one direction, then turn it 90 degrees and get it the other. If the door frame is straight, then the rest of the trailer will be straight.
After that, install the bubble wherever you want.
You will find that no two objects on the camper match to level. So you have to just pick what is most important to you to be level. Some level by the inside of the refrigerator. Some by the shower floor. Some by the top of the stove. Some by the floor. Some by the bumper.
I found the door frame is the best, because it's always installed square to the frame, even if the frame has a natural arch (which one of my previous campers did have), making the floor out of level somewhere.
So pick your spot and use that. Like I said, the door frame works well, and the inside doors will not swing either, neither will cabinet doors or the refrigerator door.