Did some more checking and I'm pretty well convinced that it's the mixer valve. Turns out from some web research, this isn't an uncommon occurrence for Atwood heaters with this feature, particularly for people running hard water through the system. Deposits get run through the mixer valve and clog it up. As I understand it, this almost always happens at the beginning of a season when people un-winterize and stir things up when filling up their heater tanks. The best defense is to make sure the sediment is cleared out of the tank when draining it at the end of the year. So now it's off to figure out my options to repair, replace, or bypass it altogether.
Fun times. If anyone's got any ideas, throw them my way.
