You might check with the manufacturer of your fridge/RV. The manual that came with our Itasca includes the manufacturer's manuals for appliances. Ours specifies within two degrees either way side-to-side and within six degrees either way front-to-back. I put a dymo-labeler tag above each level bubble (the RV came to us from a prior owner with two, in separate locations) spelling out the limits.
I always level the RV to within two degrees front to back for sleeping comfort, and am happiest when it's within one degree each way; but don't obsess over it.
I hadn't thought about the question of whether the fridge might be out of level to the RV, and haven't checked the levels since I got the RV - good point about testing against the fridge itself.
