To answer your question, a newer Volvo would definitely be a mileage improvement over an older cat. I don't have much experience with them as rvs, but with over 25 years trucking experience, the Volvo is much more economical on fuel. Other things to consider is that there are a lot of places that won't do engine work on a Volvo engine (we had a Volvo day cab with a ved12 with I-shift that got 11.8 bobtailing, 6.0 grossing 80,000lbs, and averaged 9.6 overall). With a Cat, you can get it done anywhere.