It's pretty cool to know that there are a few Trombonists on OCR.
I have been playing for 7 to 8 years. I'm in quite a few ensembles - wind band, orchestra, big band and various school bands (the 3 bands mentioned are part of my local music centre - or "academy of music and dance" as it's more corretly known as.
I have never played in a marching band, but it seems like there are marching bands all over the place in the USA. You even have competitions, don't you?
I have, however, played Fanfare Trumpet (not the standard one, but a bigger one with an embouchure very similar to the trombone (and a mouthpiece virtually the same size). I have been part of a Fanfare team on three occasions - first playing with a wind band, second with an orchestra and most recently (at a christmas concert a couple of weeks ago, in fact) with just the team on its own.
Also, on the subject of intonation, I know how sensitive it is on the tenor trombone. Over the years, I've come to discover that there are in actual fact a heck of a lot more than 7 positions on the trombone.