If you measured the AC voltage on a DC power line, you might see +33v of AC (called AC ripple). It is indeed "dirty", i.e. the DC power source is also generating a lot of AC on the same wire. Whether that's the problem is a different question. The fridge makers all spec that the DC power should be clean (filtered) but few RVs have very clean power and they still work fine. Is there a battery in this circuit? Usually the battery is sufficient to filter out most of the ripple.
In any case, try running the fridge from battery power only, i.e. no engine alternator or DC converter/charger in use. Battery power is clean, so that would eliminate dirty power as a possible cause.