We had that once. The backflush worked, got things flowing. After that we made sure we used lots of water. Never used any chemicals. That was when we were new to rv's. Now 5 1/2 yrs later. No further problems. Skimping on water use when flushing is not a good idea.
Had neighbors once with clog and bad odor problem on MH they had just purchased used. The porta potty company that will come and do honeywagon service for rv's came and flushed, cleaned and gave them something to put in to soften any remaining. Came back after 24 hrs - got more cleaned out. Did final flush . All of this cost them $75