Welcome. You can gain a lot of knowledge and information by reading the posts on this forum. I own a Class C 30ft. I have documented every gallon of fuel for every mile for the last 30K miles. It averages out to 8.5 MPG. That includes uphill,downhill, towing my Jeep 4 down or pulling a 16foot enclosed trailer loaded with racing parts.
Most come with an Onan generator that feeds off the main fuel tank, they tend to be noisy under the dinette but that's the way it goes. I use a small inverter type when just needing to charge batteries overnight .
Mine only sleeps 4,my overhead bunk is instead an entertainment center, some call it a B+.
Purchase what fits your needs at the present time. So many used units to choose from. I purchased pre-owned and haven't regretted it .