UPDATE!
I have called several RV dealers and they all say something different. I know, that shouldn't really surprise any of us.
1. Use a clear self-adhesive caulking to fill in the crack.
2. Use Versa Chem - fiberglass mesh and epoxy.
3. Eternabond aluminum tape. As mentioned by ya'll, drill 2 holes to stop the crack from running. Rub tape firmly and wait 1 hr before using tank. That is all there is to it. This is our choice.
I'll keep you posted about whether we were successful with this approach.
Thanks everyone for your input and suggestions.