Dealership tech drove screw into my fresh water tank!

Do not allow them to "fix" the tank. A repaired tank is not the same as the original tank (ask me how I know!). If they screwed up (yes, pun intended) they need to do the right thing and replace the tank with a original equipment replacement. If they will not, let them know that you will turn it over to your insurance who will then sue the dealerships insurance for much, much more money. In the end you will end up with a new tank and the opposing entities can battle it out.