I used the same approach as Charlie on a leak on the flange of the gray water tank and had good results. No leaks so far.
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If I have a similar problem in the future, I'll try a product called Plast-Aid, which I wasn't aware of at the time. I've since used it to repair a 1/8" - 1/4" hole in a PVC sprinkler line that was in a location that would have been very difficult to cut out and replace the line. The mend has held for numerous months under the pressure of the sprinkler system. I've also successfully used it to repair a leak in an air mattress. The manufacturer claims it chemically fuses with PVC, ABS and acrylic. Good stuff.
As Charlie suggested, my approach now is to dump the gray water tank ASAP before transporting the TM.