First, is the tank all black polyethylene or is it stainless steel, painted black? --Can't tell from the photo. If it's the black PE, you only have one real alternative and that is to look up a plastic tank fabricator in the phone book and get them to hot air weld the tank. In my prior life, this was the only way we found to repair cracks in PE that had any chance of holding, especially if there was any tank flex involved--and you have plenty of that, sloshing down the road.
What ever you do make sure to get ALL and I mean ALL the RTV cleaned off before starting. It has silicone in it and this will prevent anything else from bonding in that area. Thoroughly scraping, wire brushing and then thoroughly sanding might do the trick. I'd even try to hit the area with a mini-sand blaster.
After cleaning, the JM Weld might do the trick if it's a stainless steel tank.
Chet18013