Ok, so on arrival yesterday here in Las Cruces, problem recurred. I connected 50A service, all was well, had 13.7V on the readout in USE position. Turned to STORE, got 9.3V. Back to USE, had 12.5V, and the I/C fan started.
Opened up the inverter/converter cover, turned off breaker #1, and again, snugged down all 8 DC buss lugs, rather more enthusiastically this time.
After this, I switched on the I/C, and the fan did not restart. In USE position, I got 13.7V. Then, in STORE, I got 13.7V. Yippee!
It has been in STORE the last 24 hours, and I've still got 13.7V, and NO FAN.
Current best guess is I10 West from Fort Stockton to Las Cruces (parts of that road were pretty rough) shook one of the connections again.
Anyway, things are running well.