Well im 180° out, Standard pipe dope, the 'gray' stuff or any oil base will soften plastic, however there are several types of pipe joint compound suitable for plastic threads, they will say so. I use Oatey (great white w/ptfe). Rectoreseal amoung others-read the lables
Useful where tape wont seal or reusing fittings. Over torquing to get a seal (#1 failure on plastic threads is deformation) less likely than where tape is used. My first choice would be Teflon tape but in the camper 'dope', especially plastic to plastic threads or where vibration likely or thermal expansion/contraction like on water heater. All of our threaded water plumbing joints were installed with pipe joint compound, usually white. We've had no failure due to it. Weve had no leaks I assume due to it.
Coarse Ive not stayed an any motels that would give me the expertise of a professional, just my opinion. But that fitting not in line with where it should be causes me to think wouldnt have mattered what it was sealed with...