Okay, not being one who gives up easy, I checked the converter on a battery stand alone. It does work as it should, blinkers override brakes. But it still did not work on the bus. I then checked the voltage on the taillights when the brakes were on, and it was less than 9V. I have 4 taillights, so I started removing them one at a time, with the brakes on and a blinker on. When I removed the 3rd one, voila, blinker started working. I measured the current, about 2 amps per light, so my guess is that the wiring is marginally sized. I did reseat all the connectors I could find, but it did not help.
Since I had just run a heavy guage wire from the battery to power the converter and the accessory pin on the 7-pin trailer connector I added, I decided to add a relay in the rear, to activate the brake lights, instead of drawing the power all the way from the front. WOW, my brake lights on the bus are way brighter, I mean like the sun. What a difference, and the trailer brake lights work as expected.
Whew, I am glad that is over!