Joana,
My husband & I rented a 29-foot Coachman Freelander 27QB last summer and we had a wonderful trip. It has a real queen-sized bed (60" x 80") and you could walk around the entire bed. Easy to put on linens. Plenty of storage, too. The cab-over was huge and could easily fit 2 kids. The booth dinette also made into a full sized bed. The jack-knife sofa turned into a twin size bed. So, it would easily accommodate 2 adults, 2 kids and a large dog. We had 3 adults and a dog on our trip. We have a Belgian Shepard (67 lbs) and I just kept the booth dinette in bed mode the entire trip and that's where she rode for the entire trip. She loved it. She could lay on the bed and look out the window
It also had a split bath, which I like. One person could take a shower and another could use the bathroom, without being intrusive. My only complaint was the faucet in the bathroom was so short, it was right up against the back of the basin. Design flaw, in my opinion.
One of the things we liked about the 27QB is that it did not have any slide-outs. Some people love slides, some don't. It would be up to your own personal preference. We don't like slides because it's more difficult to access your outside storage when the slides are out. If you don't have slide covers, then you have to be very careful about making sure no debris from tress are in there before putting the slides back in. It can easily damage the slides. We have friends who had an RV slide ruined by debris and it was an expensive item to fix. On the positive side, slides really make your RV roomy when they're open.
As stated above, you really need to figure out what kind of floor plan works for you. Go to a few RV shows or dealers and actually sit in the RV. Sit on the toilet....is there enough room for your knees? Is the toilet up high enough or so low that you need grab bars to stand up? Is there enough height & width in the shower that you can comfortably wash your hair without knocking your elbows on the walls? Is there a real shower door or just a curtain? You have to think about where that water is going to drip off the door or curtain, which can cause damage after a while.
As far as gas mileage, most class C's that we've look at get about 8 to 10 miles per gallon. You can find some of the smaller ones on van chassis like the Winnebago Trend (Dodge Ram chassis) or the Itasca Navion (Mercedes diesel) that can get 16 to 18 miles per gallon. But then you're going to pay a lot more for them.
In all the research we've done, we've found that used RVs are a much better bargain, as someone else has already taken the depreciation hit and they've already worked out the kinks in the RV. Our dear friends who are full-time RVers and live in a 41-foot Class A also recommend buying used. Their first RV was a Class C and they said it was great for road trips, but if you're going to take trips often, they loved the Class A. They felt the engine in the Class A was more powerful, had a larger gas tank, and was a smoother ride.
We're going to be renting a smaller Class C this summer, a Coachman Freelander 23CB, since it will just be the 2 of us and the dog this year. We tried to find a Winnebago Trend or Itasca Navion to rent, but couldn't find one for a reasonable price. We definitely liked renting because we got a sense of what we liked and didn't like.
Good luck with your search & happy travels