Forgive me, but I have probably analyzed this topic to the point of overkill; I use/have used a torpedo level, those Hopkins stick-on levels, and I just bought a Camco "spider" (EzLevel) level, that I have yet to use.
Hopefully the floor should be a good indicator of the level of the RV. If the Thor manufacturing claim is correct, the floor should be aligned with the frame. Some suggest placing a level inside of the freezer compartment of the 'fridge as that is the part of the fridge that is "level" sensitive.
At any rate, the fridge and slide outs are the two items that need to be leveled. The fridge I don't think is quite as sensitive as the slide outs though.
I am not sure about Dometic and residential type 'fridges, but for the Norcold RV 'fridges, they must be within +/-3Deg side-to-side, and +/-6Deg front-to-back under operation, or it can be damaged.
I "calibrated" a torpedo spirit level for the 3deg and 6 deg to make it easy to see how level is level.
If you want to calibrate a level, I used a bit of trig to setup a ramp to do so. If you get a flat stick and measure 14 5/16" from one end, then elevate that point by 3/4", that gives you 3Deg of elevation (1 1/2" for 6Deg). I chose 3/4" and 1 1/2" for the calibration elevations as that is the common thickness of finished wood planking, so it is easy to accomplish by simply sticking a block of wood under one end of the stick.
After elevating the stick on one end, I put the torpedo on the stick and found that even at 3Deg, the bubble in the level was completely outside of the two hash marks.
Here is what 3deg looks like on my torpedo:
(I tend to photograph my projects and RV things... I'm retired and have nothing else to do)
So, my conclusion is that as long as the bubble is within the hash marks of the level, or even close, you are far under the required 3Deg of the 'fridge... at least with my $3 Menards torpedo level.
Of course, levels are not the same so you should calibrate the one you use. My conclusion though is that leveling your coach is probably going to matter more to the slide-out than the 'fridge. Even 3deg of tilt is probably going to feel uncomfortable when you sit inside.
I also have a set of those Hopkins calibrated levelers that are double-side taped to the outside of the coach:
The Hopkins Graduations are marked in 1" intervals and are supposed to tell you how many inches you are off. However, this will only work for a specific size trailer - which they do not divulge. The reason for this again is trigonometry. For any given angle of inclination, an elevation of 1" can only be true at a specific distance.
Also, I have found on my scale, that you ultimately do need to drill a hole and mount it. I have found that just using the sticky tape to mount the scale is not all that accurate as that over time, the level tends to move from heating by the sun. It probably also depends on the surface the level is stuck to as well as how open in the sun it is. But to remain accurate, some drilling will eventually probably be necesary.
Notwithstanding the calibrations, they work pretty well. I did buy a new set recently (they are inexpensive) and I think I will get around and figure out the actual distances that correspond to the 1" marks.
Lastly, I just bought a Camco "EzLevel digital level (same one JamieGeek has):
I bought it to complement the hydraulic leveling system I am having installed this spring. I have not tested it a lot, but I have found it to be really sensitive and seems to be fairly accurate.
Prior to purchase, some of the reviews I read about it was it was not accurate at all, but that does not seem to be the case from my initial impression. So much for the validity of on-line reviews. I have to test it a bit, but my first thoughts are that it seems to work fairly well.
Since the floor should be the most level spot on the coach, I plan on simply sitting it on the floor of the coach when I lower the hydraulic levelers.