Often the fittings on the water and waste tanks are "spin welded" into place. They are not removable without breaking the weld and, as far as I know, are very difficult to replace because most glues and caulks will not stick to the tank material.
Were it me, I would leave it up to a dealer or RV repair facility to do the work.
Barney
2004 Sunnybrook Titan 30FKS TT
Hensley "Arrow" 1400# hitch (Sold)
Not towing now.
Former tow vehicles were 2016 Ram 2500 CTD, 2002 Ford F250, 7.3 PSD, 1997 Ram 2500 5.9 gas engine