It looks to me like the brakes may have dived under the rim and abraded the tire.
+1, that's my first guess too.
A couple of months before each tour I go over my bike top to bottom. I've found under-tension spokes now and then. I get everything buttoned down how I like it and get a couple of hundred miles before I box it up and ship it.
I do carry a spare tire, tubes, and spokes with me. I've had a tough time finding tires and tubes on a lot of my trips, especially off-road trips. There are folding tires out there if you want something that packs down. Fully-loaded touring wears out rear wheels and tires!
I've been down the PCH in WA and OR, what a beautiful ride. We had one full day of pretty heavy rain, and I had two flat tires that day