I think I have figured out what caused the problem. I was draining my tank and now that I had the underbelly opened and could see how the fittings are arranged, it dawned on me that, even when drained, some water will sit in the fitting and the line to the pump unless I run the pump until its mostly sucking air. So, I think it froze and slightly cracked the fitting.
When I picked up the trailer and we got our walk-through, I asked what all I needed to do to winterize and one step wasn't, drain tank then run pump till its sucking air...nor was one to suck some anti-freeze into the fresh tank.
I will try the epoxy sealant approach unless someone has a different idea and hope that the swelling of the tank when loaded doesn't make it leak....and I'll be sure to run the pump after draining when winterizing.
One of these days I hope to graduate from the school of hard knocks. LOL