We gross out at 28K ( 31.5 when we tow) and have an ISC 330/950ftlbs. We have been over 12,000 foot passes in Colorado and spent most of our time in the mountain west. If you will be in the flat lands an ISB might be fine.
We traveled with friends with a newer Alpine with an ISL, on hills we were no match and his mpg was every bit as good as ours.
But remember, when it comes to pulling and pushing there is no substitute for cubic inches.
As far as mpg, IMO very few people see 10-12 mpg. I think the norm would be around 7-9, ISB, ISC, ISL. in the west.
Tom, Patty, Hannah "The Big Dog" and Abby Kat, Indianapolis, Indiana 2000 36' FDS 72232 Our Photos
We live out in our old van. Travel all across this land. Drive until the city lights dissolve into a country sky, me and you - hand in hand.