In my experience, PVC, ABS, silicon cements will not work at all, and make it difficult for anything else to work. I also suggest not using galvanize pipe at all. If metal is used, use brass.
Check with a RV supply store for the proper glue for the repair. I do not remember the name, but I used some to repair two different gas tanks, that were made oif the same material as the water tank. My suggestion would be to use the cement on the threads of the plug, and install it. You will never be able to remove it again. If it were to break in the future, you could simply drill and tape new threads.
If you can install the deck plate as bobndot suggest, and install it above the stripped thread area, it would allow you to use a Bulkhead fitting. I installed a 4-inch cover is a bit difficult to get my hand in. I suggest at least a 6-inch.