Re: 1991 Dodge Ram 350 Sportsmobile gray water tank replacem
On my SMB the greywater tank was made of ABS, I'm pretty sure. If you look on youtube (or other) you can see videos of how folks have "welded" on patches (even to larger holes like yours). I think that's what I might try, just because removing/replacing a tank can be more complicated than you'd think. And the welding looks like it works even if your skills are a bit "rough and ready."
OTOH, you could perhaps find the exact tank, and/or have a custom one made. I don't know who made the black tanks on my SMB, but my current rig has "Inca" brand tanks (there is a molded in label).