You would have to either apply it on the outside or open the tank and do it from the inside. Either way requires very good surface prep, such as light sanding, washing and then acetone wipe. Not a fun job from the inside. I've never done it before, but if you can get at it either place, I'd try JB Weld.
Don't try to fix it...replace it. You will be chasing that leak for ever if you try to fix it. Aluminum should never have been used in the first place. Urine is extremely corrosive. Metal holding tanks invariable fail at some point because of it. Try Ronco-Plastic.com. They make good quality replacement tanks.
I agree with Anthem. Whatever patch you make will fail and likely sooner than later.
My suggestion is to flush that tank as clean as you can get it, including all of the waste lines. Disconnect the tank, remove it and replace with poly. Ronco may have something that will work, or you may have to send them detailed measurements with precise location of fittings and they can likely make it.