We recently had to dig up and replace a section of sewer line leading to one of our trailers. After the plumber dug it up, he reported that there still seems to be a backup somewhere in the main sewer line. The plumber is now recommending that we jet the main line with water to try and clear it out. The branching lines leading to the trailers are made out of clay, so the main line is likely clay too. The plumber said that jetting the lines shouldn’t hurt the clay lines. Should I believe him that jetting the lines is the best course of action here, and that it won’t cause damage?