I had a significant crack in the front side of my B1700 greywater tank, presumably from road debris (gravel) hits. It had been poorly patched with some material, possibly expoxy, and a scrap piece of thin and hard plastic sheet was jammed between the tank and supporting crossmember to cover it. It leaked.
I did some experimentation, and in the end used fiberglass tape (2" wide strip of fiberglass cloth) bedded in solvent cement
to fix it. There is solvent cement for ABS and PVC piping - I got the two or three types I found in the plumbing section of a hardware store and I think I ended up with the PVC stuff (although I agree that the tank is likely polyethylene rather than PVC). These solvents seem to be loaded with some solids (they are not just volatile solvent and do build up some material) and work by partially dissolving the base material (the tank plastic). I knew I had the right stuff when I found the one which became bonded to the tank as it dried, rather than just forming an easily peeled off layer.
I built up about four layers in total, brushing on a coat of cement, laying on the cloth, then brushing it in with more cement. The solvent cement takes some time to dry, which lets you get out from under the trailer and breathe fresh air for a while (or until the next day, in my case).
The more obvious way to go is with an epoxy-based repair kit for RV tanks, as I later found in stores (such as Canadian Tire). The kits which I have seen come with the fiberglass cloth tape, too. If I didn't already have the materials, and needed to do another repair, I would probably get one of those kits.
The fiberglass cloth was important in my case, because the old crack had opened up as the edges curled inward. Mechanical strength, not just plugging a hole, was important, and just expoxy would have likely not worked. I think that this would apply to a stress crack near a fitting, as well.
By the way, Kevin, I had not noticed before that you're from my part of the world. Welcome, and get that Boler
fixed up - it's spring out there!