Below is a pic of the whole grey tank run- just as JBarca said- 2 elbows, and then out into the drain valve.
The grey tank itself is bolted into the frame- 3 bolts on each side, for a total of 6. I didn't really feel any movement in the tank itself. Felt pretty solidly attached to the frame. The more I thought on it, I do think the dips in the road on the last trip caused excessive bouncing of the trailer at 60mph speeds, and- what did the pipe in.
Where the crack is (to the right in the below pic, almost against the "collar"), there is no hub to which the pipe is going into. The "collar" is physically part of the pipe. This leaves out the option of using a boring tool to clean out any pipe in the hub (I did order the tool though. Good to have in the arsenal).
While I'm fairly certain that the Fernco coupling (hub one end, pipe the other) will fit over the "collar", once I cut the pipe, if the Fernco coupling doesn't fit, then I'm screwed.
And hence...going with the "fiber screen mesh with abs cement, 3 layers" solution from JBarca 1st. I'll drill a small hole at each end of the crack, and then go from there. Will also add a Fernco coupling (pipe one end, pipe the other) in the middle of that run to give flexibility.
In another post, saw Gdetrailer also suggested cutting an abs pipe lengthwise for a 1/2 moon, and applying that over the patch (abs cement on the 1/2 moon and the pipe, and then slipping the abs 1/2 moon onto current pipe, thus forming a bond). Potentially a next step down the road if needed.