I had a problem with my 2011 Rockwood Signature 5th wheel. The street side propane tank wouldn't feed to the two way regulator, on the curb side. When I removed the street side pigtail, to check out the one way valve in it, it was all clogged up with a sticky substance that looked like tar. The high pressure regulator (30psi) was also clogged and the hose that carried the propane over to the low pressure regulator, as well at that regulator.
Upon checking I found that all the parts that were clogged were stamped, Made in India. Now I know that India makes many high quality products but the ones used on my 5t wheel were not in that group. The best we could tell was that the pressure hoses used, were leaching some sticky oily substance out of them when exposed to propane. The same propane supplier was being used with both our RVs but this was the only one that was having problems.
The solution was to start at the street side 30# tank and replace everything in the propane system that was stamped made in India. Two new pigtails, new cross over hose, new high pressure regulator, new Marshall brand, low pressure auto switch over regulator was put into the system. Now 2 1/2 years later not a problem with the system and still using the same propane distributer.
The only contamination problems I have every had with LPG, was in growing up on a ranch in southern Oklahoma, where the distributors were prone to pump butane into your large farm tanks, for summer use. It was cheaper for them to buy than propane. The first cold snap of fall would let you know how much butane was mixed in with the propane as the butane had freezing characteristics close to the same as water. In cold weather, you could be showing a half tank and still not be able to get anything to come out of the tank.