Could a fiberglass repair kit work? I know little about the properties of fiberglass, but do know that the adhesive/bonding agent used seems to stick to a lot of things. Another possibility might be Bondo... Whatever you get to stick, if you patch both inside and outside the tank, it might hold better. You would then have to let any solvents outgas from the inside of the tank before you put your plants beck in it. A fairly thick layer of silicone calking over the whole thing might help both keep the water inside and shield the patching material from the water. Or would a patch made up of a thick layer of silicone calking inside and outside hold well enough to be a patch by itself? As you can gather, I've not repaired a Rubbermaid tank, but have a few thoughts as to how to do it, which might or might not work.

Repair that grout joint, and all you've done is set it up to crack again. Most likely, the reason that particular joint is cracking is because that's where the top of the pan membrane is. Most likely, it's time to replace the shower pan, as well as about 18"- 2 feet up the walls. One thing everyone who reads this thread needs to keep in mind-- when grout cracks, it's not the problem. It's a SYMPTOM of the problem, and 9 times out of 10, that problem is either structural movement, or moisture getting at the framing and making it swell. Either way, the problem needs to be identified and mitigated. Otherwise, you can grout it, caulk it, tape it up-- whatever. You're still going to have a problem that will need attention sooner or later, and the sooner you take care of it, the cheaper it'll be to do so.

Try a CERTIFIED chimney sweep, somebody who has completed a course of training and knows a thing or two about chimneys, how to figure out if they are okay and how to fix them if they are not.

I wish I had heard that before we bought from Aloha, AlohaCracks... as I shared on my webpage, if we had it to do over, we would most certainly buy a Fiberglass pool... just not from Aloha. Here is a link that might be useful: Detail of Aloha Figerglass Pool crack repair