I'm six weeks out from my final purchase decision and am looking to do something similar. I'm planning a six month (and later, a three month) trip with dad (me), mom, a 3-year old daughter, and occasionally, a mom-in-law.
My first choice is a Chevy-based class B towing a bunkbed 24' long trailer (21' box, expands to 29' total length). The long-wheelbase Chevy van is 20.5' long. I will be able to park in any rural or suburban area since a standard parking space is about 20'. Parking in urban areas will be difficult, since the roof height will keep me out of most garages, but if I can find street parking, I will be able to fit in a single space.
My plan is to set the trailer down in a centralized area and do daytrips out from the trailer with the class-B. Then move the trailer every week or two. I'm planning on an even mix of national parks and cities (museums, concerts, cultural events and attractions), so maneuverability is very important. In my opinion, 20 feet is the maximum you can easily park in a suburban/urban environment. At 22', you will need to hang the front or back over the curb. At 24', you will need two spaces. When you're looking at class-C's, make sure you are getting the overall length and not the box length. For example, Coachmen had a Freelander 19 model that was 23' long (19' box, 23' overall length).
The class-B is fully self-contained, so we can eat, nap, watch TV, rest, and have a bathroom during our daytrips. At night, because it's self-contained, it can serve as an office or completely detached and private living space from the trailer.
My second choice is an extended wheel base Chevy van (extended version for more interior room and longer wheelbase for easier towing). I will take out three rows of chairs (leaving seating for 5) and I'll throw in a porta-potti, a 12v DC fridge, a small folding table and some Thermarest pads. Not exactly self-contained, but it has a toilet and the ability to keep food cool. Because the van is lighter, my towing capacity skyrockets and I can tow a much larger, 28'-30' box (32-35' overall length) trailer.
A 23' box class-C is not 23' long. It will be closer to 25-26' long. A 21' box will be 23-24' long. Also, it will be 8' wide. I think the church can make sure you have a space big enough for that, but unless your destination has spillover parking or bus parking, you're not going to fit into a standard car space - unless you're the first person there and the other cars park around you. If you're looking for space in a crowded lot, no one is going to leave 8' of width for you. It all depends on where you'll be going and how precious parking is in that area. If the class C gets beyond 24', you might as well just go up to 31' because you aren't going to be able to find parking for either one outside of dedicated RV/bus spaces.
For your situation, give the class-B / van option a thought. The tow-vehicle can serve as a separate room (once set up with a table, toilet and some 12V DC appliances) and can park in places a class-C can't.
Please see if your daughter can do homework in a bouncing car. I can do work in an airplane but can't do work in a moving car/van/bus because the movement is so much jerkier.
Cost-wise, you can get a 2006-2009 Chevy van for $15,000 and then spend $15,000 on a used trailer. A 2006-2009 class B will probably run $40-50,000. Fords will have less towing capacity, but are 10-15% cheaper.
Edit: The economics. The average person uses their RV 30 days per year. The breakeven point on renting versus buying is about 90 days (less than 90, it's cheaper to rent, more than 90 and it's cheaper to buy). I'm planning on 9 months of use over 2 years of ownership. I'm figuring 10-15% depreciation in the first year, 5% depreciation in the second year with late-model used units. I'm not planning on saving any money by using a RV. By the time depreciation, gas, camping and storage costs are factored in, it will cost me the same as (or more than) living in a sticks-and-bricks house. But it will be cheaper than staying in hotels and eating out for 9 months. The biggest factor is depreciation. The older the rig, the less depreciation and the cheaper it gets.