I found out today that it is definitely my controller or my 'burb. We bought a '95 Sunnybrook TT and went this afternoon to tow it home. We had the same problems that we had with the rental. So that rules out the problem being with the rental.
When we towed the rental home today, it exhibited a different problem than going up on Friday. Today, the controller showed "c", which is normal, but it didn't apply any braking when stepping on the brakes. But it would apply braking with the manual lever. So on the way home, I just added some braking with the manual lever while stopping.
When we hooked up our TT we picked up today, it went back to applying about 1.5 volts as soon as I plugged in the trailer. I wasn't touching the brake, or even moving. To get the trailer home, I unplugged the controller and drove slowly with the flashers flashing. We took some back roads to stay off the main road, which has rolling hills and is 55 mph. Drove about 40 to 45 the whole way home (only about a 20 minute drive at normal speeds).
I'll check out the wiring harness and see if it is correct. It is always possible it is not. I got it from etrailer, so I will check with them.
Thanks. Gotta get this resolved before we go anywhere else.