510 sundance

This is my first post hope I did it correctly. I am looking at purchasing either a 2000 510 sundancer with 3196 cats or a 2000 410 sundancer with 6cta cummings. It apears the fuel usage for a 510 is 25/30gph total at 1600rpm cruse (23knots?) and the 410 is only 5 gph less, it seems like a...