End of this story. J-B Weld made the surge tank leak manageable, this allowed us to get North out of the stupid hot temperatures. Once we get back home in Eastern Wa. purchased a new steel surge tank and low level sensor. Had Cummins coach care near me install the new tank/sensor, drain, flush & fill the radiator along with our annual maintenance on engine & generator. Doesn't look like I did damage according to Cummins. Engine temp. now hangs right at 175deg F.
A happy ending (except for the wallet).