Level

I used a small carpenters level inside the fridge to set outside stick-on bubble levels. When they say I’m level I check the fridge again. Works!

If it looks level and feels level, it's level enough.

I literally attached one of these level attached to the trim on my steering column.
I found a level spot to park, verified the rig was level on my stove top, and then used double sided tape to make this match what my carpenters level showed on the stove.
Now as I back in, I select my exact parking spot according to what this shows.
Send me a PM with your mailing address and I'll send you one, I have 3 extras.

When I had a LP frig I used the level app in my iPhone and/or a round bubble level in the frig. Now that I have a compressor frig, if it feels close enough that I will not roll out of bed it is close enough. I cut the number of leaving boards in half from what I use to carry, I haven't used any so far.

I have a pair of levels calibrated in inches with one attached to the dash and the other to the driver's side panel. They let me know quickly if I'll need any supplemental blocking under my automatic jacks and how much. I leveled manually using a carpenters level before attaching them.

RV Designer E401, Stick-On Level, 2 Per Pack

And LP fridge will need to be 3 degrees or less level. 3 degrees is a lot of off-level which you will not be satisfied with, so for the fridge, if you feel it is level then the fridge is likely very happy. For you, if you are happy then it is level enough. I would suggest to get used to living every so slightly off-level if you camp a lot, this makes life easier. As long as the eggs dont run too far to one side of the frying pan you are level.

I use a T Level,place it on the floor of the freezer compartment,works for me.

Try the dollar store for the cheapest level you can find.

3 degrees is 5" drop across a 8' wide RV, good for the refer and not so good for humans. For every 12" on the level 3 degrees is 0.6".

I use a simple round bubble level placed on the floor between and behind the front seats. Easy to see from drivers seat.

I just use a simple $6 bullseye level

I try to get within half a bubble of level which is enough that the RV doesn't "feel" unlevel. I do have an absorption fridge so need to stay within a few degrees of level, but the real limiting factor is that water pools in the corner of the shower and/or kitchen sink if the RV is too far from level. (and it's opposite directions, the shower doesn't drain if the RV is tipped too far in one direction, the sink won't drain if the other direction)

I do a spot check on the dining table, then follow up with a check on the floor just case something knocked the dining room table off level.