I've used my Fastrak in my Class-B without a problem.
I just threw it up on the dash ahead of the steering wheel and didn't think anything about it. I know that when I drove in the lane previously, and did not have the transponder (we share just one), I did get a citation letter. Since the transponder beeped appropriately while I was in the lane, and I did not get a letter, then it should be working.
I used it on the I-110 south of downtown Los Angeles.