my C20 454 camper special with turbo400 & 3.73 axle & 16" tires has gotten 10-12 consistently for last 35 yrs no matter if pulling 4-horses or a 6000lb car trailer or running empty (city vs hwy irrelevant - always the same)
(& that's with both the oem cam & a rebuild @ 205,000 miles with RV-10...