What you'll probably encounter is a different playing style than you've ever seen before. Every corps hornline that's at least halfway decent spends a LOT of time and effort on achieving a well-blended, homogenous sound.
You'll have at least one person specifically in charge of the contrabasses... sorry, "tubas"... and they'll nitpick as much as possible to get you guys to play together. That person -- or "tech", as we say -- will be there in horn arc, on the field in ensemble rehearsal, and running sectionals. You must TRUST that person to tell you what's going on and how to fix things.
You'll also have people whose job is to clean up your marching. They'll chase you around the field, giving instruction on dress points, intervals, phasing corrections, foot placement, on and on. Those people need your trust, too.
So... my point?
You probably play well enough, and hopefully you have enough body control to learn SCV's marching style (there may be a typical "corps style", but the well-trained eye can spot notable differences among even the top eight). So, your skills probably aren't a factor.

You'll play damn loud, true, but you'll learn how to do it with clarity, control, and taste. You'll also learn the subtleties of every dynamic level below that. You'll learn all the ins & outs of putting a successful program on the field and how to teach any kids that you might find yourself in front of.
You won't learn a huge variety of music. But, you'll find out what a performance executed to near-perfection sounds like, and you'll be able to take that concept with you to any other ensemble.
My advice in one sentence? Shut up

Sounds blunt, I know, but it's to the point. I've only been doing this for 18 years.