First -- thank you for supporting your Girl Scout troop!
The troop leader(s) should be checking all this, not you. It's up to the GS council for your area, probably, and it's the leader's responsibility to find out. They definitely will have to get permission slips (they aren't called waivers) because any time you have an event that is outside the normal meeting space you must have a signed permission slip and it has to be the one their Council uses. This is easily obtainable on the council web site but again, the leader, not you, has to do this. As for using private property for camping -- it might be allowed, I've never asked (I'm a troop leader) but usually GS asks troops to camp at GS campsites or other "approved vendor" locations; however, our own troop camped at a state campground and it was fine. Private property may be another matter altogether because it's not a known quantity. Depending on whether there are vendor sites or GS camps in your area, you may find they allow private property camping.
Yes, GS has a lot of rules, but in the interest of keeping girls safe so their experience can focus on the activities, not on worries about safety. GS also must carry insurance to cover anything that happens if a girl is injured on any official GS outing. Hence the use of approved vendors--places checked out by the council and covered by insurance. You might be able to get your property listed as an approved location for your council! Check into that -- it might involve just proving you have certain insurance, or perhaps letting someone from the council inspect the property, or both--??
Another alternative: Have the girls and their family members all go camping together but not as a troop -- as a group of families that know each other. If anything happens, of course, I would assume that the liability would be on you as the property owner and the leader as an individual, not as a GS leader, because GS would not be involved and GS insurance would not apply. That's a big risk, though. I'd go with what the council says and keep it as a GS official event.
Girl Scouting is great and I hope your daughter enjoys it and takes full advantage of all it has to offer. Camping is wonderful but there's a lot more to do in scouting, so maybe you can offer them other help if this doesn't work out!