Ultraheat Melted Black Water Tank!

So, I come back from fishing to a prominent sewer smell. Look under the Cedar Creek and noticed a drip from the belly pan. Figured a fitting or something was leaking. Pulled the belly pan and insulation (which was a bit of a nasty job when it's full of sewage!) and start inspecting for a leak. Notice a very slight drip coming from the tape pad that holds the Ultraheat tank heater to the tank. I reach up and touch the tank heater and notice the darn thing is hot! WTH, I get out from under and go into the RV expecting one of the kids flipped the switch on and see that everything is off as it should be. Of course I grilled everyone and nobody had touched it. I went back out, snipped the wires and pulled it off to find it melted a fairly significant hole in the tank (that sucks).

This is where it gets interesting. I pull the pan for the grey tank and touch the heater and notice it's warm but not near as hot as the one I had just removed. Go back in and check the switches to see that they are indeed still off. Start checking a few things with my limited tools and decided I must a have a short somewhere and decide to just remove the 15 amp fuse that runs the tank heaters and the monitor panel. Low and behold and hour later the damn pad is still warm! At this point, I decide I better check the galley tank. Pull the pan to find that one is cold. Well this makes some sense as this heater is on a separate switch from the other two. At this point I clip the wires to the grey tank heater and taped off all of the wires to make sure I don't burn a hole in the grey tank also.

I'll be taking the multi-meter with me back to the lake next weekend to do some checking. If anybody has any suggestions where to start I'm all ears! Second, has anybody had any luck patching a tank? I am hoping to epoxy or jb weld the hole shut as a temporary fix until I can get it home to be worked on. If anyone has some advise, feel free.

Thanks