My two cents as a previous Rally host is the following:
Pick the place. You know your area better than folks coming there, so find a place you like and most folks will like it too. It should be realatively easy to get to with a truck/camper. It should be near things the group would like doing/seeing. Finding the right campground can be a challege at times. They have to be willing to hold a group of sites for the group months in advance before the rally. Plus, some owners hear "rally" and think of bikers and lots of bad images. Make sure they know the group is family oriented and not a gaggle of drunken roudies.
Pick the time. Once you know the place, then set the dates and stick to them. The date needs to be far enough in advance to enable folks to make plans/arrangments to come (with work, family, save cash, and so on). No date will work for everyone, but it is what it is. For it to happen, you gotta have a plan and stick to it.
Pick the events. Again, you know what would interest folks and what's available in the area. The key (at least in my opinion) is to have events, but not to do too much. It's just as nice to sit around the campgroud, shooting the bull with new and old friends at the firepit as it is to go on a tour, dyno, race, or what have you. Talk with owner/operators of the events/things you want the group to enjoy. Get a firm commitment and call them after the first meeting to reaffirm and recommit them/you to the group being at their place. The Rally on the east coast always has a potluck dinner and a show and shine. We give out awards from the show and shine at the dinner and it's a good time. But, it's your rally, you can do what you feel is best.
THIS I THINK IS KEY-----------> Plan the next rally before the current rally ends! While everyone is there, talk about who would host and where it'll be. More than one option... take a vote. The key is having a plan for folks to work with and it will keep going and growing each year.
Ask any questions at all from any of the previous Rally hosts. It's a fun (and stressfull) process, but worth the effort. It will be a great time and it will grow and grow each year.
Heath