Okay! I filled the black tank halfway up and....
...it still leaks.
But it's not the repair - the repair is awesome. I didn't get the resin up high enough on the side of the tank. It's really tough to get in there. I might have to grind out some metal in order to get up in there to clean and apply the resin (I have enough left to patch it). Ironically, that's not where it was leaking but in the process of preparing the area for the patch, I ground out the old patch. So now it leaks where it didn't before.
So I'm going to let it dry out for a couple of days and hit it again over the weekend. It's a tiny leak - like a drip every few seconds. But any sort of black tank leak is a bad leak.
My next trip out is in a couple of weeks, but I'll have full hookups. Worst case is I won't let the tank fill up past 1/4 (where the leak is).