I was going to mention Plasti-mend as well as a second repair product, plastic epoxy which is available from hardware stores, Harbor Freight, and others. I'd probably try plastic epoxy along with several long strips (1" wide x 18" or so) of lightweight fiberglass cloth first.
My suggestion for using it the epoxy & fiberglass would to first empty and let your waste tank dry completely out, then use 60-grit sandpaper to rough up the full circumference of the pipe.
From there and while wearing two pairs of nitrile gloves, spread your strips of fiberglass out on some wax paper and mix up a two tubes of the epoxy. Separate out about 1/4 of the epoxy and set it aside, then use the remaining 3/4 of the mix to saturate as much 1" fiberglass strip as it will cover, mushing it into the fiberglass so it is saturated all the way through. Put the remaining glob onto a piece of newspaper, then use the newspaper to mash the epoxy into the crack, and use whatever is left to goo up the entire circumference of the elbow you need to repair.
From there, wrap the 1" strips of epoxy-soaked fiberglass around the pipe. Each piece should make it all the way around once, then most of the way around a second time. Pull each strip tight and spiral your way down from above the area to be repaired to below, overlapping the repair by 1" and overlaping each wrap of fiberglass by one-half the width of the strips.
Wear goggles, wear gloves, wear a hat to protect your hair, and wear long sleeves. Uncured plastic epoxy is carcinogenic, so you don't want to get it in your eyes or on your skin. And getting plastic epoxy in your hair is only cool if you want to look like Ken from Barbie and Ken fame.