emissions on california class c's

I don't think that the "smog" air pump takes much power to run, they spin easily by hand, they run a long time before failure with no maintenance. Most 450 V-10's have about the same fuel economy depending on tire pressure , winds, RV weight including water tank, stuff and people, as well as speeds above 55mph, grades vs flat land driving, altitude, and to some extent use of cruise control vs manual throttle and use of gears all affect fuel economy which varies from 7-11 mpg based on user comments that I have read. RV's are large non-streamlined, heavy vehicles with lots of rolling resistance. Diesel powered RV's cost considerably more new, diesel fuel itself is fairly expensive, and the RV must be driven many, many miles for the fuel economy pay off. Periodic diesel service is fairly expensive also.
If you can afford owning and maintaining an RV, cost of fuel really becomes apparent on long trips with infrequent stops, otherwise it becomes acceptable. If you eat sandwiches, etc.,in the RV, not in restaurants, you can offset fuel costs quite a bit