So the vet vet he saw when he was sick while I was gone threw another potential diagnosis into the mix. Radiation induced neuropathy. It is a thing in people, can happen anytime either during or after radiation (as in 2 years, 10 years, or 30 years). It often results in a dropped foot while walking in people, which Gavin doesn't have (they test for it every time we go in). It can manifest as loss of feeling, tingling, or pain, etc. It does appear that Gavin does not have significant feeling in his foot at times, but his reflexes appear normal when tested. When standing, he puts almost no weight on that leg, just barely resting it on the ground. He didn't lose any muscle tone while I was gone as Anne did walk him quite a bit, but his limp is the same. Sigh...at least he enjoyed his trip to the beach today! The only thing I found online that may help neuropathy is CBD oil, so I ordered that, but it won't actually improve his condition but may lessen pain/tingling and/or prevent further decline. Apparently chemo induced neuropathy can be reversible but radiation induced isn't.
Once he gets started on his walks, he really moves quite fast considering his limp. When I get back from my vacation, my goal is to work on improving his gait (and hopefully his limp) if I can.
I knew post radiation neuropathy was a possibility, but all the papers I read had it as a rare complication. I am coming to realize that even in human cancer treatment, it is very common to downplay the risks with the attitude being "You're alive, aren't you?" and very little research in how to address these complications. The utter despair you can feel from posts in support groups from people suffering and getting no help or support is very sad. 
