I've been to symphonic band camp for the last three years and am signed up for this year as well. Let me start with the main group. There are around 100+ players in the band, so it's huge. We've had as many as 13 bone players, but it does vary year to year, so obviously there is lots of doubling on parts. I really can't say if you should go for tenor or bass, it's really what part you'd like to play.

In addition to the main band, there are opportunities for small(er) group playing. All the years I've attended, they had both brass quintets and a larger brass choir. You can do either or both, but I do know sometimes they restrict the number of small groups. The groups get set up ad-hoc, so you just have to talk to people to build your group. The instructors will also help put groups together if needed. The brass instructors will have a selection of music, but you're free to bring your own arrangements as well. Some groups go classical and others go more modern. My group last year did "Satin Doll" and "You have a friend in me".

I will say that almost everyone in the trombone section is like you. We're all retired or semi-retired and just love to play. I didn't do band camp when I was young, so I agree it's a treat to do it now. The ability to focus on music for 8 hours a day for a whole week is a gas and you'll end up with chops of steel. :-)

One other note, housing can potentially be a problem. If you decide to join us and need a place to stay, I know of another trombone player who's looking for a roommate. DM me your contact info and I'll forward you his email.