I'm in a similar situation - dad, mom and a 2 year old daughter. The car seat is do-able. The only manufacturer that has LATCH, the standardized hooks that car seats attach to, is Winnebago. I'm not sure which year they started providing these. They will be in the forward facing dinette bench.
Without LATCH, you will have to use a seatbelt. Unfortunately most class-Cs only have shoulder belts in the front two seats. This means that you must have a car seat that works with only a lapbelt. Most new car seats are designed to work with a shoulder belt or LATCH.
Also, car seats are designed to work forward facing or backwards facing (relative to the facing of the vehicle). If you mount the car seat on a sofa, the car seat will be side-ways facing. Most acccidents will be from the front or back, so with a side-ways facing child seat, the child will experience a side impact collision. There are car seats which have extra padding to limit head movement in a side impact collision. A car seat that works with only a lap belt and has extra side impact protection is the European Britax. http://www.britaxusa.com/car-seats/advocate-70-cs
Could I offer another option? I'm looking at towing a small travel trailer with a class-B. I will "base camp" out of the travel trailer and use the class-B to drive to the attractions. A 20.5' class-B will allow me to easily park almost anywhere and the small travel trailer will let me use state parks. During the day, we will have a self-contained class-B where we can prepare our own meals (cheaper and healthier) and our daughter can nap. At night, the class-b will serve as a separate office where mom or dad can get some work done while the other parent sleeps with the toddler in the trailer (maybe a good baby monitor will let us both work in the class-B). We'll move the trailer once a week or so to a new location and explore the new area. This will keep hitching and unhitching down to a minimum, give us lots of living space and maximum flexibility to explore.
The cheapest route for my plan is to buy a 15-person van second-hand. In Orlando, the hotels and tour companies swap out their vans every year or two. You can get a clean, late model (1 or 2 years old) Ford E350 with about 30-40,000 miles for $18,000. The 20,000 milers are about $21,000. Take out the back three rows of seats and you have room for a porta-potti, small electric fridge and airbed and 3000+ lbs CCC. The 2nd row bench will have LATCH and shoulder belts for the car seats because it's a late model passenger van. This kind of van could be used as a daily driver too, saving even more money. http://mullinaxfordfl.com/Orlando/For-Sale/Used/?MakeId=12&ModelId=97&Model=Econoline%20Wagon&Make=Ford
My wife looked at pictures of a porta-potti and said "NO". So now I've got a deposit on a late model Chevy class-B.
If you go the class-B route, check to make sure the aftermarket shoulder belts have locking pretensioners. Roadtrek, for example, does not put locking pretensioners on their 2nd row captains chairs. If the shoulder belt won't lock down, it won't hold the carseat in a fixed position. Some car seats will come with a special clip that will lock the shoulder belt. However, you may have to search for a seat with a compatible locking clip as almost every car in the USA today has locking belts and many safety seat manufacturers have stopped providing the special clips.
Safety-wise, a forwards facing car seat (a two year old will probably be too big for a backwards facing seat) mounted facing towards the front (not sideways) is the safest option. I feel that the metal body of a van is safer than the wood frame of a class C. The vans all have poor rollover ratings, but are vastly safer than a motorhome if they roll. Vans and class Bs have to pass FMVSS specifications for their seats and seatbelts. Vans and class Bs have to pass passenger vehicle safety standards. Motorhomes don't. The late model vans also all have computer sway control (note, class B vans are usually based on the cargo van, not the passenger van, so sway control may be optional on those).
EDIT: I've rented class Cs motorhomes (the larger 30+ ft ones), on my previous trips. The Ford V10 will easily hold 75mph, but the handling is not stable at that speed. I mostly did 60-65 mph (or less). Gas mileage never exceeded single digits. I think I saw 7-9 MPG.
EDIT 2: For maneuverability, look at overall vehicle length (the manufacturers misleadingly use box length in their naming nomenclature, for example, Coachmen had a Freelander 19 model that was 22' long). The average parking spot is about 20' long. Parallel spaces about 24' long. You can get a 22' truck/van into a 20' space if you park the front or rear overhang over the curb (don't block the sidewalk please). At 23' or more, you will be sticking out of a single space even parked over the curb. Nothing will fit in a parking garage because of roof height.
Edit 3: The OP said he liked to do very light off-roading. Quigley makes 4x4 conversions for the American vans. http://www.quigley4x4.com/Home.aspx
Edit 4: I had a 2003 Dodge Ram 2500 with the Cummins. Made yearly trips to Daytona Beach towing 2-4 motorcycles in open and enclosed trailers. Small mountainous terrain over the Appalachians on I-75. Also did the trip with Chevy Express 3500 with 6.0L gas engine and Dodge 350 van with the 5.2L gas engine. Never took any of the Fords. The 5.2L was adequate - you could easily maintain 55mph upgrade, but if you slowed down, it would take a long, long time to get back up to 55mph. The 6.0L gave you passing power upgrade. The Cummins didn't even notice the hills. I guess it's up to you. Any of the gasser V8s will do the job, just depends how much acceleration you want going upgrade (diesel had better compression braking downgrade but the gassers were adequate). Get a V10 Ford if you need more power.