how to level a trailer

I don't think it matters as long as you get it close. I use the trailer tongue because it's close to the electric jack.

Fridge.....

Cooktop

In my Class C RV I use one of These on the floor of the freezer compartment of the fridge. Less than half a bubble off each way,almost always dead center both ways when I am finished.

When I bought my TT , checked the fridge and compared it to the tongue by the jack. I checked both for/aft and left/right. They were very close so I check at the tongue. It beats running inside or having one of us inside and the other one using the electric jack.

I just use a bubble level app on my cell phone and set it on the counter right above the mini fridge.

Two stick on bubble levels at the right front corner of the trailer, one North/South, one East/West. Worked fine, no muss no fuss.

I guaranty that if you stick a level in more than one place on your RV that you'll have more than one different readings.
When I set mine up for the Levelmate Pro that I installed I just used the floor in front of the sink. No matter where I stick the level I get a different reading.

I'm a lucky guy -- I am married to The Bubble. She just steps into the trailer and consults Her inner level.

It's only level enough when She says it is. ;)

Seriously, though, there is a lot of variation from place to place in the trailer. We finally decided that the middle of the floor was the best place. We then mounted these things to the outside of the trailer, calibrated to correspond to that spot on the floor.

So that way, we can level up without ever having to check inside the trailer.

Until and unless, of course, She enters the trailer and decides that it is not quite right. ;)

Level the trailer front to rear and side to side as perfectly as you can by using a long level on the frame, then install a pair of levels on opposite corners. Now you can put the long level away and use the installed levels in the future knowing that they are correct.