I have a 454 in my Class C. The RV weighs about 2 tons. I get 6 to 8 mpg.
But it's a good engine. I have gotten the Class C up to 80 mph on I 20... not the smoothest road. I don't think I could have stopped it too quickly but the motor seemed to like being pushed and I probably could have run it faster. Because of the stopping distance required (and the fuel sucking capabilities once I get into the back 2 barrels of the carb), I rarely put the Class C over 65 mph. But the short time of the faster run seemed to help the motor since it sits more than it runs ( a common occurrence with most RVs). I had gotten hung up in traffic getting back onto the interstate and had to run to catch up with David & Stacey. If you don't mind the poor MPGs and it will swap in, why not. Keep in mind, it probably has sat more than it has run.