the area around the fitting on the bottom of my aft water holding tank
had been broken It had been "repaired" by the prior owner with gobs of silicone and was leaking. I found a repair method online that worked great and has lasted for two years now. I bought a polyethylene cutting board at walmart for a buck something. cut out the damaged area and cut a plug
out of the cutting board a little smaller leaving about a quarter inch gap. Then welded a fitting into the plug
and welded the plug in using a pencil torch and some high heat glue gun sticks. Its very similiar to welding with a set of torches. just form a puddle on both surfaces and add the glue gun stick in like welding rod untill the desired mass is reached. it turns out the glue gun sticks are ethylene and make a very professional weld. It worked for me.