Brett,

I've had great luck patching water and holding tanks in my RVs using Marine
GOOP and Fiberglas webbing.  Once it sets it is impossible to remove.  I
suggest Marine Goop over any other kind as it has UV protection.  Other
kinds will deteriorate (in my experience.)   Clean the crack area.  Spread
some Goop on (I use my fingers) and quickly imbed a piece of Fiberglas
webbing (auto parts store) in it.  Spread a little more Goop on and make
sure you press it in firmly so as to dislodge any bubbles that might be
there and assure good contact with the surface underneath.  This stuff will
be fully cured in 24 hours and you'll never have to worry about it again.
If it does leak due to a bubble or some such that you didn't get fully
sealed, simply add some more Goop on top.

Scott