We have a large pack as well and 30% attendance is about the normal for campouts. But 30% attendance is like around 20 families which means around 60-80 people with parents/siblings included.
I don't think its a bad turnout considering when you have to compete with parents schedules, other sibling schedules, sports and kids getting sick etc, and of course people who just don't like to camp out.
I think the biggest key is communication which gets harder and harder the larger pack you have. Get to know what parents read emails, need to be texted or called to be reminded of events. I usually sent out at LEAST 2-3 reminders for each event through the method I know the parents will see.
The "Parent Meeting" idea posted by SeattlePioneer is an *EXTREMELY* good idea and could generate more involvement from parents who don't know more involvement is needed/required. Make sure you have concrete things that the parents can volunteer to help out with. One example that we involve parents in is the campfire planning meeting. When we do a campout, usually on Saturday night we do a campfire with skits/songs/etc. Put someone in charge of it and get a few parents to volunteer to help out, attend a campfire planning meeting and help come up with songs/skits/activities to do at the campfire. The biggest thing is that when someone steps up and says they want to help, have something asap that they can take charge of and own.
Just make whatever the event is the best event you can for the kids that showed up. You cant get disheartened about attendance because of all those factors I listed above.