I'm not sure what you mean by "Water tank". If you are talking about the fresh water tank........ that tank is made of polypropylene. NOTHING...... and I mean NOTHING will truly adhere to polypropylene except polypropylene welding with a good plastic welding machine and polypropylene welding rod.
If you are referring to the waste water tank.......... That tank is made of ABS plastic. It can also be "welded" but it is essentially a "chemical weld".
I have done extensive chemical welds on my current TrailManor Elkmont black water tank and I posted pictures and the procedure on this forum. You should be able to find that post (quite long and detailed) with a search of "tank repair" and my name "Wavery".
To answer your question about drilling holes at each end of the crack........ When a crack happens in plastic (indeed most materials), the crack will get progressively worse as the crack weakens the material at the end of the crack until that material separates, continuing the crack. When you drill a small hole (usually 1/8") just at either end of the crack, you remove the weakened material, thus ending the progression of the failure. Then when you "weld" over that area, the crack and holes are all filled to sound material.
