You may have to downsize more than you think. Currently, the Ford Chassis (E450) which is the most often used, has the greatest load carrying ability, 14500 pounds. Older Fords, may not be able to carry that much. Once the weight of the "home" portion of the RV is put on the chassis, you will be lucky to have the capacity to carry 3000 pounds. From that number (the Occupant and Cargo Carrying Capacity) you need to subtract the weight of yourselves, your pets, your clothes, your food, pet food, tools, and other stuff. With a Class C, remember it is not how much space or room you have, but how much weight you can safely carry. Generally speaking, the longer the Class C the heavier it is, and options like slideouts and leveling jacks also decrease the amount of weight you can carry. Towing is an additional weight factor as is whether or not a particular car is towable.
That said, I know couples who full-time or nearly full-time (9 or 10 months/year), who do quite well with a 27 foot Class C with no slides.
Carefully research your plans. There are some full-timers who blog and their reports can be read on-line. Be very objective when running the numbers. As noted by others, RVing is a fun hobby/lifestyle, but expensive.
Best wishes for success with your decision.