Scott,
There is nothing that I know of that you can apply to the tank to stop the leak. The tank probably has several cracks in it that can open up as the tank expands under pressure. You can plastic weld a patch over the area, but it probably will not last very long because of the heat and pressure, especially if you are still running the original 13 PSI radiator cap. As I have stated on the forum several times before, a 7 PSI cap will extend the life of those plastic tanks greatly.
You can fill the tank and loosen the cap so that it does not hold pressure and drive that way if your coach does not run over 200 degrees, but since I am not sure exactly how your coach is configured, I do not know your operating temperature. You listed your coach as a Patriot Thunder with a 3126. The Thunder came with a C12 that normally runs cool enough to drive without a pressurized system, but the Patriot 3126 usually runs a little hotter, so that may cause an issue. What ever you do, be very careful no to overheat the engine. Just pull over and let it cool down if you see anything above 215.
Gerald