[LHaven]

Well, that did it. I closed the bottom valve, did about 400 miles, used the toilet once on the road and once as soon as we set up camp, and it flowed full force. It's definitely hot water heater sediment migrating backwards out the cold feed.

I'll flush the sediment when I get to our destination, then I'll look at inserting a check valve into the cold feed (it may be a challenge, as that loop is way the heck in there).

Thanks to all who helped narrow this down.