Could you just drill out the screws and then replace them with a size or two larger? That is a quick fix and works great if it is a Phillips type screw because the drill bit won't wobble as much. OR, can you take a pair of vice grips and grab the head of the screw, assuming it is not recessed? It is difficult to picture the exact nature of the support but if they are badly stripped, I have even resorted to the tried and true of just prying a bracket apart with a large screw driver.
It truly sounds like you are going to have to drop the tank to fix the problem because you have eliminated every other possible source of the fluid. If the fluid starts leaking when you fill it only an inch or two, I bet it had more water in it over the winter than perhaps you thought and just froze laterally enough to crack the tank.
Most of us including yours truly have had naughty winterizing experiences at one time or another. I am a little paranoid and I drain my lines and tanks, then I blow them out, then I fill them with about three times as much antifreeze as most suggest. My theory is that rv antifreeze is cheap and for ten bucks extra I can just feel better about it. I won't bore you with how I came to that conclusion on my first motorhome a long time ago. But it wasn't very pretty!!!
Good luck,
Paul
