Though I can see DeeDee's point about how avoiding stimuli can often make anxiety worsen, I think it is fair to note that this is a potentially prolongued anxiety-provoking situation with potentially no built-in support available. I really sympathize with your concerns.
Is your dd able to express what exactly about the trip makes her anxious? I would absolutely work with her counselor to try to help dd clarify this: is it just being away for two days, or having a panic-attack without support nearby, or what the bathrooms will be like?
Once she can clarify this (if she is able to have that insight), then the counselor can help you determine if this is an anxiety that can be managed with a set of 'tools' she can use or if this anxiety is severe enough that it could cause further harm (ie: teaching one who's scared of water to swim by throwing them in the deep end).
If the latter is the case, I would most certainly have the counselor write a letter (backed up by her PCP...sometimes schools get a little dismissive of those without an MD behind their name).
I would also make sure that there is a plan in place with an adult who is on the trip if dd's anxiety increases which may include things like deep breathing, spending extra time with adults or call(s) to home. You can even pre-write notes of comfort/encouragement that dd could open when she is getting overwhelmed.
...and I totally understand what you mean about kids who 'hide' their anxiety. They often do a marvelous job at appearing "normal"...and suffer deeply on the inside.
