UPDATE: I was able to repair it thanks to all of you! I tested it in my driveway after a week of drying, then we took it camping and it’s still not leaking
I’m so happy! It was over $1000 to replace the tank. Someone local offered to do the “plastic welding” for $350. I was able to do it for around $60 (it would have been $30 if not for the inferior ABS pipe I started with). Thanks to all of you for your help, and patience with a fresh rookie in this game!